It looks like you are trying to open a Prime 9 file with Prime 8. This can't work as PTC has decided for incompatibility between all versions even though the file extension remains the same. While Prime 9 can open a Prime 8 file, it cannot save a file in Prime 8 format like other software is able to do (losing just the features not available in the older version) and your P8 refuses to open a file created with a newer version.
In former versions of Prime it was able to copy the whole file (you may have to put it into an area first) from Prime X to an older version of Prime if you have both installed, but this has been broken and now only one single region would be allowed to copy. So copying one region after the other is rather prohibiting even if someone has Prime 9 and Prime 8 installed.
@LucMeekes seems to have found methods to convert files down to older versions - I'm not sure how and how much effort it takes.
If you can post the P9 file he might be able and willing to convert it for you.
I often used a simple method posted by a user here in the forum to open newer files with Prime 6, but the method wasn't really reliable an ever so often fails. But if you post the file I could give it a try or you can give it try yourself - here is the link
https://community.ptc.com/t5/Mathcad/Prime-backwards-compatibility-solved-Or-at-least-hacked/m-p/637537
