Does the XML validator I mentioned above find any issues in your files?
For converting Excel to ReqIF, I posted instructions in another thread on this forum at Importing ReqIF file via xlsx2reqif python code conversion, example files are included there.
If you’re comfortable with scripting, there’s also another way to get ReqIF for Capella – your scripts could produce ReqView project files in JSON directly. The JSON structure is quite simple, have a look at the example data there. You could just open these files and then export ReqIF.