You are definitely not the only one, see
- ModuleNotFoundError: No module named 'limesdr'
- python - ModuleNotFoundError on GNU Radio right after installation - Stack Overflow,
- gnuradio error · Issue #480 · ptrkrysik/gr-gsm · GitHub
It’s quite common problem with gnuradio 3.8 as the directory where the OOT modules gets installed when installed from source is not in the default python module search path (sys.path)