Fatal error: Could not check compatibility

Could you please add some steps to reproduce here, that would be helpful?

It seems like that interface file is missing.

It appears that you have patched PageTitleBlock somehow, but then also missed out a use statement, because there is no blockForm() method defined in the core version of PageTitleBlock; there also isn't a line 112: https://git.drupalcode.org/project/drupal/-/blob/10.2.1/core/lib/Drupal/...