Install JDK from unity hub

I would try closing the unity hub (make sure its not minimized, the X from the program doesnt close it) and open it again as admin.

You can download manually the openjdk and in the Preferences - External update the path.

A third option would be, if you have 2 versions of unity installed and you already have downloaded the jdk for that unity version, you can update the path of the jdk to both unity version point to the same jdk

Navigate to the Unity Download Archive. Find the version of the Editor you need from the list and click the green Unity Hub button. The Hub opens and displays the Add Unity Version window. To install any additional modules along with the Editor, select them from the list shown and click Install. My Ascension Portal