The reason why you get a message saying "Wrong Pocket" after potting a ball means that you have not potted the nominated ball in the nominated pocket.

In "Pro" rules, you have to nominate the ball and pocket that you wish to hit and pot into, if you pot a ball in a pocket that you have not nominated for it to go into, the player that you are playing against will get his/her turn.

When you see the word (P) next to a players name, this means that they have enabled PRO rules for their games.

Regards,
сяосоdіlє