Why would 'Het Appel' be wrong? Which I guess it is, since 'De appel' was right.
Appel is always a De-word. This never changes - the best you can do is memorize which words are De-words and Het-words as there is no real rule to working it out. Check out: https://www.duolingo.com/comment/3732938
Wow, was it really that simple? Thanks!