Uiche is also evening (in the previous answer), so why is uiche not accepted here?
tráthnóna is afternoon/evening. oíche is 'night'
Yes but maidin go hoíche is a phrase, you'll never hear someone say maidin go tráthnóna.
If someone is asked to translate “morning to evening” correctly, then that person should say maidin go tráthnóna. The exercises here are not exclusively focused on translating common phrases.
Tráthnúna or tráthnóna - both spellings have been used - are they alternatives?
Your post is the only example of "tráthnúna" that I can find on Duolingo. The only example that I can find of it anywhere else on the Internet is clearly an OCR error.
"tráthnúna" isn't an alternative, it's a typo.