Hi! I have exactly the same issue with the last one (cash register) which I finished this morning (I had already completed the others through legacy curriculum, is it the same for you and the ones that “Show Solution”?).
I see here: I am not able to claim my Certificate that we are far from alone in this situation and that “This is a known bug that is being worked on” .
A solution was proposed that did not work for me but maybe for you! 