Why not: "a meat and a fish"?
You can't say "a meat" in English, it's uncountable.
No article "a" (which means "one") for "bulk" nouns, like meat or water,
and for the nouns which are just "plural" by nature, like: fish, or sheep.
Is this referring to "fish" generally, or specifically one fish ("a fish")?
Whether it is one or two, or just meat, it is always fish.
The noun fish is plural, so it never needs the article "a".
When you go fishing, you catch one fish, two, three fish.
When do you use "ryba" and when "rybe"?
"Ryba" is nominative - mianownik (kto/co?), "rybę" is accusative - biernik (kogo/co?).
Just look at the examples:
To jest ryba. It is a fish.
Mam rybę. I have got a fish.
Duolingo has it wrong.
To jest ryba - It is fish
Mam rybę - I have fish
What, suddenly "a fish" doesn't exist? One fish = "a fish".
Is ę prounounced with a sort of en sound?
Roughly speaking, yes, except at the end of a word (or in a final -ęl or -ęł).
Have you tried the English "w" (Polish ł)
after the Polish "e" in the word "mięso?
Mięso - Mie(w)so - (Miełso) ----> Mięso
Why shouldn't fish be ryby in this translation, i thought with ryba it needs to be "a fish"?
It says "ryba", so it is "fish": Mięso i ryba - Meat and fish
For "ryby" it is also... "fish": MIęso i ryby - Meat and fish
"Mięso" is cognate with "member" (limb).