Hello @lcicerale , you cannot join an on-premise Windows Server machine to Azure AD. In order for your on-premise client machines to be able to benefit from the license distributing software installed in your server/DC you will need to remove your computers from Azure AD, join them to your (on-premise) Active Directory/Domain Controller and finally, enable Hybrid Azure AD join. For more informatoin about the latter please take a look to Plan your hybrid Azure Active Directory join implementation and Configure hybrid Azure AD join.
Let us know if you need additional assistance. If the answer was helpful, please accept it and rate it so that others facing a similar issue can easily find a solution.