The tool requires a Spatial Analyst or 3D Analyst extension license. The conditions where a license may not be available include the following:
- The extension was not installed.
- The extension was not enabled.
- There is not a license available to be checked out for use.
- Install the extension. Consult the ArcGIS installation guide or contact your system administrator.
- Check the "Licensing and desktop administration" section of the help for assistance with activating the extension.
- If Spatial Analyst has been enabled, other users on your network may be using all the available licenses. You could see if someone who is not actively using the extension can close their ArcMap session. If this is a frequent occurrence, you could consider acquiring more licenses.