So, I sent a report, but I think I may have erred on this one. Sorry about that if so.
The report I sent was to say that "a fish" needs adding to the database since we're accepting "the fish", and that furthermore "fish" (without an article) must be incorrect if fish isn't an uncountable noun, as that'd refer to the plural in English.
But now I realise... ryby is in fact plural, which is why "...fish" and "...the fish" are accepted, but "...a fish" is not?
If it's quick and easy to add a pop-up note saying "ryby is plural" (when correcting the "a fish" translation), that'd be a good reminder for any who make the same error.
EDIT: I've just received an email notification indicating that my suggestion "You are delivering a big fish" is now accepted. So... Is "fish" uncountable in Polish and/or has the same singular and plural forms?
No, that would be a shoal/school of fish (if they are alive, of one species, and swimming together), or some other descriptor "...of fish", such as "a tank of fish", "a plate of fish", "a delivery of fish", etc, depending on their status otherwise.
"A big fish" means only one fish.
(Or, in the metaphorical sense, a person who is too great for their environment; short for "a big fish in a small pond")
"Ryby" is indeed Singular Genitive, but more importantly, it is Plural Nominative. And Accusative.
"duże" is both neuter singular and plural not masculine-personal. For those genders, Nominative looks the same as Accusative.
Tips can be misleading sometimes, unfortunately. They are made for the whole course, not the specific sentence that you see.
The verb "dostarczać" takes Accusative, so that makes it clear that the fish are plural. If you delivered just one big fish, that would be "Dostarczacie dużą rybę".