In Ukrainian it's necessary. We separate sentences with distinct "grammar bases" (subject-verb couples). "Він любить" is a separate one, followed by a clause sentence "коли прохолодно"
The problem here is that it's period (full stop, ".") instead of comma (",")
Yeah sure :) but you asked whether a comma is necessary, so i answered that :)
(I guess you meant "Isn't a comma necessary?")
Again, as in the previous sentence 'He likes when it is chilly' is incorrect English where I live - He likes it when it is chilly.' is how we would say this. If this is correct grammar for other English speakers, which it may be (I don't know), then at least both options should be given/ accepted please.
Also, in both this and the previous question (he likes when it is cloudy) there is no option to report this point in the selection of things to report that we are given.
I'm not sure what you mean, but usually there is a report button. It appears after you've entered the exercise and your answer got rejected. If it's not displayed, it might be a bug. In that case I would appreciate it if you could attach a screenshot here.