I ran into the same problem trying to install Turbotax 2021 Desktop to amend a tax return.
The problem is that the License Key Window is not expandable.
After entering your License Key, you will need to double click in the Window to activate the vertical and horizontal scroll bars.
Scroll down to the bottom of the Window and you will see a Tab that says 'Activate License'.
Took me a few minutes to solve this puzzle.
