Recently while enabling a user in my Dynamics CRM 2016 OnPremise I got below error :

Root Cause:

The error clearly tells me that there is definitely something wrong with the user setup on Active Directory. After digging in further this is what i understood as what exactly might happened with this user :

1. User was added to AD
2. User was added to CRM
3. User was disabled in CRM
4. User was deleted from AD
5. User was again created in AD

Conclusion:

Now in CRM whenever you create a user (applicable to AD authentication) it stored an object GUID of the AD account with the database column as ActiveDirectoryGuid in systemuser table.

final Conlusision : so now when you try to enable the user again – CRM try to get this object GUID from AD but it will not find it, hence will give you this error.

Solutions:

1.  Get the new GUID from Active directory and update it in CRM database[ systemuserbase table and ActiveDirectoryGuid column] – Have not tried it personally. 
2. Create a new user account in CRM for this user – it will allow you since there would be no GUID conflicting. [you might get error as duplicate record exist – you can try to put some addition middle name of character in the full name ]

What I did:

i have in this scenario used the second approach as the user was fairly new and there was not much dependency with the record.

However if a user very old and have a lot records in CRM – you will have no choice but to go for the first step.

Hope this helps!

Cheers!