SharePoint 2013 Troubleshooting: The User does not Exist or is not Unique in
Issue:
The user does not exist or is not unique in SharePoint 2013 when we check the user in people picker.
Solution:
This issue could be for many reasons, In this article relevant solutions to resolve the issue are provided.
Solution 1:
Ensure that there is no space. The most common issue that occurs with the above is inadvertently adding a space character between the elements of the path.
For example: instead of OU=Users,DC=CONTOSO,DC=COM you typed OU=Users, DC=CONTOSO,DC=COM
Solution 2:
Ensure that the user account is not in a Disabled state in AD.
Solution 3:
Ensure that the user account is not in a Locked out state in AD.
Note: If you find a different solution, please report it as a comment to this post. This will be useful to the SharePoint Community. Be sure to double-verify it: undo your solution and verify that the problem comes back, then redo it and verify that the problem goes away.