are you sure that the file you want to open is indeed an IDML file?
Maybe someone sent a file that just was renamed to suffix *.idml and it is something else entirely?
If that's not the case and you saved an IDML from an InDesign document, just try to open the InDesign document from a previous version in your updated InDesign 2021 version 16.2.1.
If you like make the IDML file available through Dropbox or a similar service and I try to open it with my German version of InDesign 2021 on Windows 10. Just post a download link. Do not attach the file, because you will not be able to change your answer after posting.