Symptoms

When starting an application session in the Workspace, an error is shown: 

A terminal license server could not be found to provide a license

Cause

On the Microsoft Windows Server RDS infrastructure, there is an issue with the terminal license server.

Mind that this is related to Microsoft licensing. It is independent of Parallels Secure Workspace licensing.

Resolution

Source: Activate the Remote Desktop Services license server | Microsoft Docs

Activate the license server

1. Open the Remote Desktop Licensing Manager: click Start > Administrative Tools > Remote Desktop Services > Remote Desktop Licensing Manager.
2. Right-click the license server, and then click Activate Server.

Some organizations have a group policy where the license server is explicitly defined.
In such cases:

1. On the RDS host, open the Local Group Policy editor.
2. Navigate to Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Licensing.
3. Verify Use the specified Remote Desktop license servers and check if the correct server is specified.