Hello! If you go to the "Availability" tab in ArcGIS Administrator, are their licenses available there? If so, how many licenses are listed in the "Licenses" column? How many in the "Available" column?
Hey aysunçınar! Just to add to Megan's comment, what type of license are you using? Is it a single use license or a concurrent use license? If this is a concurrent use license do you know what version of License Manager you are using?