Apprentice licenses are valid for 30 days at a time. When the expiry date is approaching, you will receive notifications when starting Houdini that that the Apprentice licenses are expiring. You can re-activate the licenses every 30 days.

In Houdini 21.0 the License Administrator interface has changed.

Using the Houdini Launcher, select the Licenses menu on the left then click on the License Administrator button.

Navigate to the General menu on the left side in the License Administrator. Click the Activate Apprentice button.
Ensure that the license server is set to the local machine and not the SideFX server. In the Servers menu on the left side, ensure it is set to either "localhost" or the local machine name.

To remove Apprentice licenses, please refer here