I can’t see the whole error message but it goes wrong at: Checking Unity 2021.2.8f1…
Why don’t you use UnityHub for keeping track of different installs?
I received this same error when trying to install via the 2021.3.16f1 Download Assistant.
I can confirm that installing instead via the Unity Hub link (available from the Download Archive: Download Archive) solved this problem… whatever it might have been!