The extension could not be activated. License request failed. All licenses are in use

Error Message

When trying to activate an ArcGIS Desktop extension license in either ArcMap or ArcCatalog, the following error is displayed:

Error:
Extensions
The extension could not be activated. License request failed. All 3D Analyst licenses are in use.

Note:
This article pertains to ArcGIS versions 9.x. Later versions of ArcGIS may contain different functionality, as well as different names and locations for menus, commands and geoprocessing tools.

Cause

All available licenses for that particular ArcGIS Desktop extension are in use.

Solution or Workaround

To work around this issue:

• Try to access the license again later when a license becomes available.
• If no other users have that particular extension license checked out, restart the license service. For more information, refer to How To: Release a license that has stopped responding from a 9.x License Manager server.
• If the license has not been checked out and restarting the license service does not release the license, verify that there are no expired licenses. For more information, see Error: Warning: The (extension) license has expired.