Volume can't be detached

I would check your Personal Health Dashboard and see if there is info about your instance being degraded. Probably a good idea to open a ticket with support to have AWS check the instance. I have seen similar situation with detaching ENI and attaching to replacement instance in the past when there was an issue with the physical host that the instance was running on. This usually resolved itself after some time (about an hour) but AWS was able to force the instance to stop and allow the detachment via support ticket.