If you receive a message on your phone that says Could not add card it likely means someone else's iCloud account is active on your phone and a different user has already issued a credential using the same iCloud account. It could also mean that your Digital ID is loaded to a different (old) phone or Apple watch. You'll need to revoke that card or create/login with a different iCloud account.
If someone else's AppleID is active on your phone, you should create a new one for yourself. After you do that, you'll be able to add your Miami Digital ID to your personal AppleID.
If you already are using your correct personal AppleID, then you may need to remove/revoke the incorrect card from the wallet by following these steps: