We’ve had lots of reports of this welcome screen issue and there’s a work round.
The button is simply off the bottom of the window at your display scaling, so:-
Go into Windows Settings and set the display scale to 100%.
Run CCleaner 7 and get past the Welcome Screen.
Set your display scale back to what it was.
Also you may well want to refer to this later:
How to Roll Back from v7 to v6.39