%! TeX program = lualatex
documentclass{scrbook}

usepackage{amsmath}
usepackage{cleveref}
usepackage{luatodonotes} % can be resolved by using todnotes
% usepackage{todonotes}

begin{document}
begin{align}
   a&= b & c &= d
  label{eq:elastic_moduli_relations}
end{align}

end{document}