Translation:I can't see a big fox but a big dog.
While it's certainly true that direct, literal translations often don't work out so well, I don't think that's a problem here. There's absolutely nothing wrong with the English sentence. Since DL allows for multiple translations, some alternatives may be accepted, too.
it should be translated "I don't see a big fox", and I can assume that because in Serbian, 'ne vidim' means both "I don't see" and "I can't see", and Serbian and Czech are similar languages. it depends on the context, but in this case it should be translated "I don't see" because it implies that the person sees a dog instead.
"I don't see" and "I can't see" normally can be differently translated in Serbian and that's why I wrote 'and', but forgot to explain that "I can't see" has two meanings, thinking that further discussion about that laguage is irrelevant here considering that this is a Czech lesson. But thank you for your explanation.