First, you will need to do is make sure the reviewer's VA User Profile in Viewpoint is filled out correctly. You will need to make sure that the PR Co and the Employee number are filled in.
Second, you will need need to check a few portal settings. Go to Admin > Portal Settings > Review AP Invoice.
This setting is used if you want to give reviewers access to use AP Unapproved Invoice Review without giving them access to any other module in the portal. If this is unchecked, you will need to make sure the reviewer has been assigned to at least one other module on the Admin > User Access page of the portal.
This setting allows you to restrict your user counts and which reviewers have access to review invoices in the portal by creating a HQ Reviewer Group in Viewpoint. If this is filled in, you will need to make sure the reviewer you are giving access is in the HQ Reviewer group listed in this setting.