It should be allowed. You can report it.

Apart from the fact that it's plural Genitive ('mieć' = 'to have' takes Accusative in positive sentences, and most (all?) of those that take Accusative in positive, turn into Genitive when negated), what else is a problem?

Declension of 'czarny' as an adjective

Negation. accusative changes to genitive in negated sentence.

Masz czarne skarpety- nie masz czarnych skarpet.

As Immery said. Adjective gets inflected alongside with the noun and negation causes them both to be in genitive case. Here you can find full declension of the word "czarny": https://en.wiktionary.org/wiki/czarny

Yup, definitely the accent/tone or w/e shouldn't be there on '-pet'.

Well, there was plural used in the sentence, and Genitive plural (needed by "nie masz") is "skarpet".

"skarpety" could be Genitive singular (Nie masz czarnej skarpety), one sock.

And of course "skarpety" is Nominative plural, and also Accusative plural.

The Polish sentence doesn't specify that it's about wearing socks. Nie masz czarnych skarpet either means that you don't own any, or you just don't have any with you at the moment.