You're getting the Warning "This file was created using a file type definition that does not exist on your system for example "Word 2007 v 2.0.0.0" when opening an SDLXLIFF in Trados Studio 2021.
The following functionality will be restricted or unavailable:

• Saving 
• Verification
• QuickTags
• Preview

Translate the file and send the return package back. The project manager who sent you the original package will be able to create the target file.
If possible, please recommend they use the new Word 2007-2019 file type when creating new projects, this is available in both Studio 2019 and Studio 2021 and works way better than Word 2007 v 2.

Alternatively, you can check the solution from the KB https://gateway.sdl.com/apex/communityknowledge?articleName=000002445

The package you received was created in an older version of Studio using a legacy word file type names Word 2007 v 2.0.0.0.
This file type was no longer introduced in Studio 2021 so certain functionality like save target file is not available.