documentclass{standalone}
usepackage{siunitx}
usepackage{tabu}
usepackage{lmodern}
usepackage{textcomp}
begin{document}
begin{tabu}{rS}
  US  & $5.00\
  CAN & $6.50\
  UK  & textsterling4.00\
  EU  & texteuro4.50\
end{tabu}
end{document}

! Extra }, or forgotten endgroup.
__siunitx_table_column_end:n ...ne_fill_skip -#1}
                                                  tex_kern:D c_zero_skip ...
l.14 end{tabu}

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