If you want to use the email address you are currently trying to use, first log out, and then back in with the old address. All cameras and sync modules will magically appear.
Then, delete all cameras and sync modules from that account. That will release them and free them up for registration with the new account.
Once the old account is empty, choose log out again, and then log back in with the new account that you were trying to use. This time when you try to add your items, it will go fine, because they will now be unregistered.