It's likely that the file contains "real" whole rests rather than "default" whole rests which are required to make MM Rests. Select all then run the plugin "Note Beam and Rest Editing > Change to Default Whole Rests". That should allow MM Rest creation.
Certain measure functions (double bar lines, rehearsal letters, certain tempo expressions) create multi-measure rest breaks. Not sure whether you can remove these with the Measure Tool. I do not even try.