RID Pool Depletion

I have this type of warning below:

Why the warning appeared when creating user by using Active Directory Users and Computers.?

The warning is appeared because they are running of identifier pool where RID block size contains up to 500. by default, each domain is allocated 500 RIDs. when it comes to 500, the DC try to contact FSMO DC to request another 500 RIDs.

you would also see the warning on Event Viewer. Please take note the Event ID 16645