Shalom, Can one study Talmud alone, and reach the level of extracting or grasping the final Halakha, even if he has never studied with a Rabbi?
is there a halakha that forbids this?
In general, one cannot come to know the final halacha from merely learning the Talmud. It needs additional understanding of the Rishonim, etc. Shulchan Aruch and Poskim, and the knowledge of how to pasken, which is usually learnt from a Rebbi. The reason for this is because there are things that mentioned in the Rishonim, explaining the gemora that the person will be missing. The same applies to learning the Shulchan Aruch, which was written after learning the Rishonim, when there are differences in opinion, so how do pasken. Even after that, there are the poskim who came after the Shulchan Aruch. Although there are times that we get the psak straight from the gemora, it is not the general rule.