Why is a Vault Owner unable to add a new user to a Vault?
If the Vault has not reached its maximum limit for user licenses, this is likely caused by the existence of a duplicate user record.
Two users having the same First Name and Last Name values creates a conflict in Vault and could produce a server error.
To resolve this error, the name of one of the users needs to be changed so that they no longer match.
Vault User and Groups Documentation: Creating and Managing Users