Site left blank - requester

Site left blank - requester

WHen I imported all the AD users into SD we encounter a problem. Some users were not able to log in and the system would give them an error similiar to "The user is no associated to an account, site or region". 
So, what I did is go into the requester's edit form and found out that the site was left blank.
When I click on the drop down menu nothing show there.

How can I solve this issue?

Kind Regards
Julian

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