You will need to contact support with your serial number, and proof of ownership. If it was previously registered to your account, they should have a record of it from when it was previously in use, but if not, your proof of ownership will do the trick. They can clear the flag, and allow it to be registered again.
It is to prevent items being usable if stolen. Not sure why it has gone wrong if it was previously deleted, because that should have removed it from the register. I’ve deleted things on a whole bunch of occasions, and it’s always worked, but that doesn’t mean there can’t be glitches.