KB: プライベートライセンスは席が埋まっているため使用できません
Solution Details
If all seats of private license are occupied inadvertently, an administrator can still log into the server machine directly where the on-premise server software is installed (Vault/Nexus Server/Concord Pro/AIS/AES) either locally or via remote desktop to release the seat through a web interface. Type the address 'http://localhost:9780' in a web browser. Default username/password are: admin/admin. Expand Admin - Licenses, select the license and click 'Release'.

Remotely accessing the server requires a CAL (Client Access License), which is occupied most probably, so please make sure you are at the server's machine where the server software (Vault/Nexus Server/Concord Pro/AIS/AES) is installed.
For further details, please refer to manual pages for respective on-premise server software:
AES : Enterprise Server Licensing
AIS : Altium Infrastructure Server
Concord Pro
Nexus Server
Vault