"Cha toil leam an t-ubhal."
Translation:I do not like the apple.
Because the masculine definite article prefixes t- to the nouns starting with a vowel and ubhal apple is a masculine noun in Gaelic. That’s explained in the tips and notes for the Food 2 skill. You can read the tips and notes on the web browser version on Duolingo (unfortunately not available on the mobile apps).
You can also access the tips on the duome.eu website.