When I try to make a request to https://lichess.org/api/challenge/open, it returns a 404 html page.
#Open-ended challenges not working
muted rampart
ripe condor
You can read about the endpoint at https://lichess.org/api#tag/Challenges/operation/challengeOpen
One initial guess is that you sent a GET request instead of a POST request... Do you know about those two methods?