In theory, that simply means that the defaults you're trying to delete can't be deleted because they're not there at all (probably because you already deleted them). You can check that by running:
defaults read com.bohemiancoding.sketch3 disableAutomaticSafeMode
which should output something like
The domain/default pair of (com.bohemiancoding.sketch3, disableAutomaticSafeMode) does not exist
The real question is: does Automatic Safe Mode work after you restart your computer?