Which delete key?
The only view in Scrivener that doesn’t allow deleting is when you display a snapshot in an editor (for that matter, you can’t edit them when displayed in the inspector or anywhere else either). If you’re editing anything else, then Scrivener isn’t at fault for not allowing the delete key to work.
What version of Scrivener and what version of Mac OS are you running? If you’re using a very old version of one, and a very new version of another, it’s possible that may be the reason you’re experiencing this very odd bug. Otherwise, it may be some other setting or application on your Mac interfering.