Could not connect to an ArcGIS license manager running on host "Not_Set"

Error Message

When launching ArcMap, the following error returns:

Error: 
Could not connect to an ArcGIS license manager running on host "Not_Set"

Cause

This error occurs if a license is not authorized or when the location of the ArcGIS License Manager is not specified after authorizing ArcMap with a Concurrent Use license.

Solution or Workaround

Authorize ArcMap with a Single Use license

ArcMap is authorized with either a Concurrent Use or Single Use license. Single Use license authorization is performed on the machine where ArcMap is installed and requires an authorization code from MyEsri. Refer to ArcMap: Authorizing your software for more information on authorizing ArcMap for single use.

Note:
Administrator rights on the machine are required to select a Single Use license in ArcGIS Administrator. In Windows, right-click ArcGIS Administrator, and click Run as administrator.

Set the license manager location

If the error occurs when using a Concurrent Use license, set the location of the license manager in ArcGIS Administrator. Refer to Error: "Not_Set" is an invalid hostname. Please enter a valid hostname. for more information.