Duolinggo, please use "lelah". Because "capek" is a slang word and not formal in Indonesia language.
Dia tidak lelah
Capek
Kenapa ditambah ed?
Ya iya lah, klo tire doang mah artinya ban.
not always 'lelah', but can use 'kelelahan'
Kelelahan is not a noun? But here I am + adjective.
Lelah = capek
capai = not accepted
padahal sama menurut KBBI :
https://kbbi.web.id/capek