If-condition cannot be used in multline environment?

It is presumably an issue with grouping. If you enclose the aligned environment in braces the code compiles:

documentclass{article}
usepackage{amsmath}

begin{document}
begin{multline} % <- Replacing `multline` with `equation` can fix the problem.
  iftrue {
    begin{aligned} h end{aligned} % <- Removing `aligned` can fix the problem.
      }
  fi
end{multline}
end{document}