Why I attacked my farm and loot zero gold?

Check if you are using the class legion, only the class legion can take gold.

Also check if your farm account is a Farmer class. Farmer class accounts have depot protection which can safeguard some of the resources when attacked. (other classes may also have a small protection level from the alliance tech) If your farm account has a higher depot protection level on the gold than the amount they have available then you cannot take it.