RSA Product Set:  SecurID
RSA Product/Service Type:  SecurID Access Prime

1- Inviting a user to RSA throw Error: Invite User Operation failed: Unresolvable Principal with userid already exists in realm: xxxx01 (UserID)

2- Checking the Unresolvable users found there are huge number of unresolvable users which could be having the same unique identifier of the user as from the Identity source side it's removed so UUIID can be shared with another user but on the AM it's still not removed.

Make sure you take snapshot.

1- Open the security console.

2- Go to : Setup > Identity sources > Clean Up unresolvable users ( Check for last 7 days and further).