You can translate it to Don't believe that.

The sentence is missing the ne word, it’s a bit in sms language if you want.

Adding to the answer of Ayfri on the meaning, which is correct:

Il faut is not only used for an external need or an obligation. It can be also used for strong advice or recommendations, as is the case here.

Ex:

Ne l’écoute pas, il ne faut pas croire tout ce qu’il dit.

Il faut vraiment qu’on pense à réserver nos places.

Il faut arrêter de croire que tes notes s’amélioreront sans travailler.

Il ne faut pas jouer avec mes sentiments, je suis quelqu’un de sensible.

So il ne faut pas croire ça means "you shouldn’t believe that". An alternative could be ne crois pas ça or the more idiomatic ne va pas croire ça. But using il faut means the advice does not apply only to the interlocutor, but that it is a more general piece of advice that can apply to anyone.

"Tu ne peux pas vraiment croire ça" has a different nuance: it would mean there is a factual, objective reason to not believe that. It is not just some advice you are giving: you are forcing your interlocutor to acknowledge a fact. It can even imply you doubt your interlocutor is sincere.

Ex:

Il t’a dit que la terre était plate ? Mais tu ne peux pas croire ça.