Question: Hello! Could anyone explain to me why this was wrong h

hello, it should be "na" and not "ng". "Masarap ng pagkain" will not make sense, unless you were trying to say "Masarap ANG pagkain" (The food is delicious). Moreover, "na" is a ligature connecting the adjective "masarap" and the noun "pagkain" to create the same sentence, "The food is delicious".

Beginner here. You are saying delicious food, I assume.
Adjectives and nouns are joined together with one of two ligatures; na or ng. If the first word ends in a consonant (as yours does), na is used. If the first word ends in a vowel, ng is used. Na is a separate word, whereas ng is added to the first word.
If however you are saying the food is delicious, then masarap ang pagkain would be correct (where 'ang' would be translated as 'the').

If I were to directly translate this in English, "masarap ng pagkain" would be "delicious of food". And for "masarap na pagkain", it would be "delicious, that is, food". Maybe this will help you make more sense of the difference between "na" and "ng".