documentclass{article}
usepackage[utf8]{inputenc}
usepackage{csvsimple,longtable,booktabs}

begin{filecontents*}{table7.csv}
A,B,C,D,E,F,G,H,I,J,K,L,M
Germany,GDP,X,X,,,,,,,X,,X
France,GDP,X,X,,X,,X,,,X,,X
end{filecontents*}

begin{document}

csvreader[
  respect all,%
  longtable={|p{2cm}|p{2cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|},
  separator=comma,
  table head=caption{List with data.label{tab:mytable7}}\
    toprulebfseries A &bfseries B &bfseries C &bfseries D &bfseries E &bfseries F &bfseries G &bfseries H &bfseries I &bfseries J &bfseries K &bfseries L &bfseries M \ midruleendfirsthead
    bottomruleendfoot,
  late after line=\,
]{table7.csv}{1=fieldx,2=fielda,3=fieldb,4=fieldc,5=fieldd,6=fielde,7=fieldf,8=fieldg,9=fieldh,10=fieldi,11=fieldj,12=fieldk,12=fieldl}{fieldx&fielda&fieldb&fieldc&fieldd&fielde&fieldf&fieldg&fieldh&fieldi&fieldj&fieldk&fieldl}

end{document}

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{array}
usepackage{csvsimple,longtable,booktabs}
renewcommand*{arraystretch}{1.4}

begin{filecontents*}{table7.csv}
    A,B,C,D,E,F,G,H,I,J,K,L,M
    Germany,GDP,X,X,,,,,,,X,,X
    France,GDP,X,X,,X,,X,,,X,,X
end{filecontents*}

begin{document}
    You simply wrote verb|12=fieldl| instead of verb|13=fieldl|.

    But vertical rules with verb|booktabs| don't go well together.
    If you want vertical rules, don't use verb|toprule|, verb|midrule|
    or verb|bottomrule| (and increase verb|arraystretch|):

    csvreader[
    respect all,%
    longtable={|p{2cm}|p{2cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|p{0.3cm}|},
    separator=comma,
    table head=caption{List with data.label{tab:norules}}\
    hlinebfseries A &bfseries B &bfseries C &bfseries D &bfseries E &bfseries F &bfseries G &bfseries H &bfseries I &bfseries J &bfseries K &bfseries L &bfseries M \ hlineendfirsthead
    hlineendfoot,
    late after line=\,
    ]{table7.csv}{1=fieldx,2=fielda,3=fieldb,4=fieldc,5=fieldd,6=fielde,7=fieldf,8=fieldg,9=fieldh,10=fieldi,11=fieldj,12=fieldk,13=fieldl}{fieldx&fielda&fieldb&fieldc&fieldd&fielde&fieldf&fieldg&fieldh&fieldi&fieldj&fieldk&fieldl}

    But it is much better to get rid of the vertical rules:

    renewcommand*{arraystretch}{1.2}
    csvreader[
    respect all,%
    longtable={*{2}{p{2cm}}*{11}{p{0.3cm}}},
    separator=comma,
    table head=caption{List with data.label{tab:norules}}\
    toprulebfseries A &bfseries B &bfseries C &bfseries D &bfseries E &bfseries F &bfseries G &bfseries H &bfseries I &bfseries J &bfseries K &bfseries L &bfseries M \ midruleendfirsthead
    bottomruleendfoot,
    late after line=\,
    ]{table7.csv}{1=fieldx,2=fielda,3=fieldb,4=fieldc,5=fieldd,6=fielde,7=fieldf,8=fieldg,9=fieldh,10=fieldi,11=fieldj,12=fieldk,13=fieldl}{fieldx&fielda&fieldb&fieldc&fieldd&fielde&fieldf&fieldg&fieldh&fieldi&fieldj&fieldk&fieldl}

end{document}