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begin{table}[htbp]

    caption{Tool/technique for scope definition}

    %begin{adjustbox}{width=1textwidth}

        label{tab:fir}

        centering

        begin{adjustbox}{width=15cm, height=5.0cm}

        begin{tabular}{|p{3.5cm}|p{2cm}|p{2.5cm}|p{2cm}|p{2cm}|p{2.5cm}|p{.5cm}|}

            hline

            multirow{2}{*}{textbf{Techniques/tools}} & multicolumn{6}{>{centering}p{1textwidth}}{textbf{Combination of different approaches used as monitoring and controlling newline methods in Software Project management}}\  cline{2-7}

            &Monitoring & Quantification & Controlling & Visualizing&Completeness & Ref \



        hline

        Earn Value Management and Gantt Chart & $1$ & $0$ & $1$&$1$ & $0$ & cite{sillitti2011agile} \

        hline 

    Earn Value Management and statistical Methods & $1$ & $0$ & $1$ & $0$& $0$ & cite{levin2009fundamentals} \

        hline 

    Earn Value Management and Kanban & $1$ & $0$ & $0$ &$1$ & $0$ & cite{corral2011evolution}\

        hline 

        Value Stream Mapping and Kanban & $1$ & $0$ & $0$ &$1$  &$0$ & cite{hans2013work}\

        hline

        Scrum and Kanban & $1$ & $0$ & $0$ &$1$ & $0$ & cite{wnuk2009feature}  \



        hline 

    Control charts & $1$ & $0$ & $1$ &$1$ & $0$ & cite{sanghera2008fundamentals} \

        hline 

    PRINCE 2 and Balance Scored Method& $1$ & $0$ & $0$ &$0$ & $0$ & cite{sanghera2008fundamentals}  \

        hline 

        Cumulative Flow Diagram with some metrics&$1$ & $0$ & $1$&$1$ & $0$ & cite{sanghera2008fundamentals} \

        hline 

    Earn Value Management and critical path method & $1$ & $1$ & $1$ &$1$ & $0$ & cite{sliger2008software}  \





        hline 

    EVM and Use Case Point& $1$ & $0$&  $1$& $1$& $0$ & cite{wnuk2008visualization}  \



        hline 

    end{tabular}

end{adjustbox}

end{table}

Table line is missing at the top right corner, plzzz help

Using

multicolumn{6}{p{13.5cm}|}{centeringtextbf{Combination of different approaches used as monitoring and controlling methods in Software Project management}}

would fix the error and add the missing vertical line:

However, the resulting table looks distorted, due to the use of adjustbox. To make the table fit onto the page, I'd suggest the usage of tabularx. In order to save space, you could rotate the table column headings as follows. I have also shortened the text in the multirow, as I would try to add the information into the caption of the table. Additionally, I have removed vertical lines and replaced horizontal lines by booktabs' rules:

documentclass{article}

usepackage{geometry}



usepackage{multirow}

usepackage{tabularx}

usepackage{booktabs}

usepackage{adjustbox}

usepackage{array}



newcolumntype{R}[2]{%

    >{adjustbox{angle=#1,lap=width-(#2)}bgroup}%

    l%

    <{egroup}%

}

newcommand*rot{multicolumn{1}{R{45}{1em}}}



newcolumntype{Y}{>{centeringarraybackslash}X} 





begin{document}



begin{table}[htbp]

    caption{Tool/technique for scope definition}



        label{tab:fir}

        centering

     begin{tabularx}{textwidth}{l*6{Y}}

        toprule

        multirow{2}{*}{textbf{Techniques/tools}} & multicolumn{6}{c}{textbf{Approaches}}

            \  cmidrule{2-7}

            &rot{Monitoring}  & rot{Quantification} & rot{Controlling} & rot{Visualizing} & rot{Completeness} & rot{Ref} \



        midrule

        Earn Value Management and Gantt Chart & 1 & 0 & 1&1 & 0 & cite{sillitti2011agile} \



    Earn Value Management and statistical Methods & 1 & 0 & 1 & 0& 0 & cite{levin2009fundamentals} \



    Earn Value Management and Kanban & 1 & 0 & 0 &1 & 0 & cite{corral2011evolution}\



        Value Stream Mapping and Kanban & 1 & 0 & 0 &1  &0 & cite{hans2013work}\



        Scrum and Kanban & 1 & 0 & 0 &1 & 0 & cite{wnuk2009feature}  \





    Control charts & 1 & 0 & 1 &1 & 0 & cite{sanghera2008fundamentals} \



    PRINCE 2 and Balance Scored Method& 1 & 0 & 0 &0 & 0 & cite{sanghera2008fundamentals}  \



        Cumulative Flow Diagram with some metrics&1 & 0 & 1&1 & 0 & cite{sanghera2008fundamentals} \



    Earn Value Management and critical path method & 1 & 1 & 1 &1 & 0 & cite{sliger2008software}  \







    EVM and Use Case Point& 1 & 0&  1& 1& 0 & cite{wnuk2008visualization}  \



        bottomrule 

    end{tabularx}



end{table}



end{document}

The code for the rotated table headers is borrowed from here: https://tex.stackexchange.com/a/32690/134144

Don't use adjustbox to make tabular material fit inside the textblock, unless you really wish to risk making the font size so small as to make the material unreadable without a magnifying glass. Instead, load the tabularx environment, set its width to textwidth and allow line breaks in the five header cells.

For the following screenshot, I've set the variable textwidth to 15cm as that's the value you use in your code for the width of the adjustbox.

documentclass{article}

usepackage[english]{babel}

usepackage[textwidth=15cm]{geometry} 

usepackage{tabularx,ragged2e}

newcolumntype{C}{>{Centeringarraybackslashhspace{0pt}}X}

usepackage[skip=0.333baselineskip]{caption} % optional



begin{document}

begin{table}[htbp]

setlengthtabcolsep{4pt}       % default: 6pt

setlengthextrarowheight{2pt}  % for a more open "look"



caption{Tool/technique for scope definition}

label{tab:fir}



begin{tabularx}{textwidth}{| >{RaggedRight}p{4cm} | *{5}{C|} c |}

hline

    Techniquesslash tools & 

    multicolumn{5}{>{hsize=dimexpr5hsize+8tabcolseprelax}C|}%

    {Combination of different approaches used as monitoring 

     and controlling methods in Software Project management} & Ref. \  

    cline{2-6}

    & Monitoring  & Quantification & Controlling 

    & Visualizing & Completeness   & \

    hline

    Earn Value Management and Gantt Chart 

      & $1$ & $0$ & $1$&$1$ & $0$ & cite{sillitti2011agile} \

    hline 

    Earn Value Management and statistical Methods 

      & $1$ & $0$ & $1$ & $0$& $0$ & cite{levin2009fundamentals} \

    hline 

    Earn Value Management and Kanban 

      & $1$ & $0$ & $0$ &$1$ & $0$ & cite{corral2011evolution}\

    hline 

    Value Stream Mapping and Kanban 

      & $1$ & $0$ & $0$ &$1$  &$0$ 

      & cite{hans2013work}\

    hline

    Scrum and Kanban 

      & $1$ & $0$ & $0$ &$1$ & $0$ 

      & cite{wnuk2009feature}  \

    hline 

    Control charts 

      & $1$ & $0$ & $1$ &$1$ & $0$ 

      & cite{sanghera2008fundamentals} \

    hline 

    PRINCE 2 and Balance Scored Method

      & $1$ & $0$ & $0$ &$0$ & $0$ 

      & cite{sanghera2008fundamentals}  \

    hline 

    Cumulative Flow Diagram with some metrics

      &$1$ & $0$ & $1$ & $1$ & $0$ 

      & cite{sanghera2008fundamentals} \

    hline 

    Earn Value Management and critical path method 

      & $1$ & $1$ & $1$ &$1$ & $0$ 

      & cite{sliger2008software}  \

    hline 

    EVM and Use Case Point

      & $1$ & $0$ & $1$& $1$ & $0$ 

      & cite{wnuk2008visualization}  \

    hline 

end{tabularx}

end{table}

end{document}