The mobile site of the freecodecamp is very buggy. I can’t type anything in code section of a challenge. I tried checking view desktop version checkbox of browser too, but that doesn’t solve the issue either.
If you are using a mobile device, the default keyboard does not always play well with the freeCodeCamp editor. I recommend you read the article on using a mobile device to view some tips - most importantly, check out the coding keyboards listed as they interface with the editor well.