Apprentice will need to be activated every 30 days.
You cannot use the Login function within the License Administrator to manage licenses.

In Houdini 21.0 the License Administrator interface has changed.

Using the Houdini Launcher, select the License Administrator at the bottom.

Navigate to the top right corner drop down menu where it says "No Account" and select "Use Enterprise Servers".

Do not select Login from this menu option in the License Administrator as this is reserved for Indie/Commercial/Education users.

Once you have select "Use Enterprise Servers", remove www.sidefx.com from the top section "Search these license servers", if listed.
This can be done by pressing the X button beside the entry.

Add your local machine (Local Server) by pressing the + button and the Update button at the bottom.
If www.sidefx.com is listed in the top section "Search these license servers", remove it and add your machine to the list (Local Server).

Next, navigate to the General menu on the left side in the License Administrator. Click the Activate Apprentice button.

To remove Apprentice licenses, please refer here