Sometimes because of the vagaries of how iPhones work, the same phone will show up as several “different” devices if you do things like update the OS or factory reset. We try to let multiple of the same phone show up in most cases, but there are some where the existence of a device on the list will block that same device from accessing your household. Confusing? Definitely.
But long story short, what you’ll want to do is just keep revoking from the list until you’re below the limit, after which you should be able to add your phone back again.
For example, if you’re on the Free plan (limit 2 devices) and your iPhone SE 2020 shows up 10 times, if you delete 9 of those listings, you should be able to add it back again.
You can read about this here too: I revoked my old phone, but I still can't add my new phone to Goodbudget? | Goodbudget (sometimes a slightly different wording helps)