It’s a bug: Crossfade Clips fails when Audacity is set to Deutsch (German) · Issue #5848 · audacity/audacity · GitHub
I posted the fix here but unless you are familiar with editing Audacity’s translations your best option will be to work around the issue.
Workarounds:
- Change the Audacity language settings. (English should work)
- Downgrade Audacity to an older version.