'Select an Eligible Reviewer' Error message for User Access Certifications

Summary: When an approver Assigns a user access request to a reviewer, the list shows a whole lot of people, however when he selects a person , it says - 'Select an eligible reviewer'.

Content (please ensure you mask any confidential information):

When an approver Assigns a user access request to a reviewer, the list should show only the eligible users who can be assigned as reviewers. Why is it showing a whole list of people and when the approver selects them it says 'Select an eligible reviewer'. Which role/priviledge will let a person become an eligible reviewer?

Version (include the version you are using, if applicable): 25C