Why not "We do not remember what we have done in April"?
When the exact time or interval of an action is specified, you cannot use Present Perfect.
Absolutely no issue with this sentence, but ... I translated this and was like, "oh, so Duolingo introduced a new lesson for talking about pandemic lockdown experiences". :D
and then i hope you saw the comments from 2 years ago.
we can add sentences to the live course, but it takes a new tree to add whole new "skills" (the things with the round icons).