This error may occur if one of the users being merged has Course Requests or an Application. Additionally, a Candidate can only be merged with other Candidates, not Students
To determine why you are unable to merge duplicate users:
- Core > User/Access > Profile
- Edit User Profile Data
- Search the New User
- Scroll Down to System Information
- Click Remove User
- Once Remove User is clicked it will provide the message and tell you why the User cannot be deleted. For example:"To permanently remove this student, all school registration rows must be deleted first. An alternative is to withdraw him/her and assign the Past Student role."