documentclass[12pt,tikz]{standalone}
begin{document}
    begin{tikzpicture}
        coordinate (a) at (0,0);
        draw (a)--(0,1);
        draw[xshift=2cm] (0,0)--+(0,1);% works;
        % draw[xshift=2cm] (a)--+(0,1);% not works
    end{tikzpicture}
end{document}