Cannot is a single word, not a combination of can and not. It should be "We cannot do what we wanted to do." Since melakukan is used, I believe "could not" would be even more appropriate.
What's the difference between apa and apa yang?
is there any way to go back to a lesson or level you have completed?
Yes, you can continue playing any level, just click on it. But they become easier again, sadly, because the multiple choice questions appear again.
There was no option for "mau" (want) only "did"