This is expected behavior.
By default, if the vIDM user is not imported, when logging into Aria Operations, the user gets a Read-Only permission.
However, this behavior can be disabled from:
Administration -> Global Settings -> Allow non-imported VIDM user access.
When you disable this option, VIDM users will not be able to login until the user account is imported from:
Administration->User Accounts
When selecting the VIDM source, Aria Operations redirects you to the vIDM server to authenticate.
As soon as you login to VIDM, Aria Operations checks whether this user has been imported.
If the user is imported, the user gets the role assigned during the import.
If the user is not imported and the "Allow non imported VIDM user access" option mentioned above is enabled, the user logs is able to log in with read-only access.
Otherwise, the user gets the error message that a user import is needed
If the behavior you describe is not wanted, then disable the setting from:
Administration -> Global Settings -> Allow non-imported VIDM user access