! Undefined control sequence.
<argument> c_sys_engine_xetex_bool

documentclass{scrreprt}
usepackage{siunitx}
usepackage{fontspec}
begin{document}
    Some Text
end{document}

AtBeginDocument
  {
    @ifpackageloaded { fontspec }
      {
        int_const:Nn c__siunitx_degree_int    { 176 }
        @ifpackageloaded { unicode-math }
          { int_const:Nn c__siunitx_micro_int { "003BC } }
          { int_const:Nn c__siunitx_micro_int { "000B5 } }
        int_const:Nn c__siunitx_angstrom_int  { "0212B }
        int_const:Nn c__siunitx_ohm_int       { "003A9 }
        bool_if:NT c_sys_engine_xetex_bool
          {