Error "Email Already in Use" (but it's not)

I had three users/licenses for my account. I deleted one user/license today. User 3 needs to take over for User 2 with the same email they used when I had a license for them.

When I try to change User 2's email to former User 3's email, I receive the error that the "Email is already in use," but it is not in use.

How do I change the user emails.