in English, an illness does not heal, patients heal! An illness also does not get well, the patient does. You could say "He is not yet healed from his illness" or "He has not yet recovered from his illness ", but "His illness is healed" or "His illness is recovered" are strange. If it is correct Indonesian, I can't think of a good direct translation in English.
Duolingo, in the drop down menu of translations, gives 'get better' as a translation of sebuh. Why then, when i give my answer (as we would say in English) 'the illness has not got better' is it incorrect? Duolingo, you can be really sloppy, which is unacceptable as a linguistic app. Why dont you have more native English speakers checking your translations?