"It is not a port, it is a farm."
Translation:To nie jest port, to jest farma.
Yes, it is. Unless you decide to go with 'tylko', which is something similar to 'but', but it's more like... you're correcting someone's claim.
Literally that would be: "It is not a port but a farm", but it is 1)way more likely to be spoken by a native and 2)close enough in meaning; so I think you should report it next time as "my answer should be accepted".
You could also say "To nie jest port, tylko farma". Literally, "it is not a port, rather a farm."
I don't understand why it is not correct to write "to nie jest portem to jest farmą"
Let's focus just on the second part: "It is a farm". You cannot use Instrumental in such a sentence. It would be as treating "it" as some kind of a noun, but it's a dummy pronoun. It's a "This is Y" sentence and such sentences use Nominative. Same applies to the first clause, it's just negated.
More info here: https://www.duolingo.com/comment/16373167