after getting on the boat to Minsk I am at Minsk dock but the guard says I don't have permission to enter the city

may i know, what is the current main mission ask you to do?

If you want to enter dorgania (by going through minsk), you have to progress the game first to the point where the main mission ask you to find a wizard  in amagal. To do that, you need to have chiyo. you can find her in manastyr. Find a way to the underground (i can't tell you in which room the stairs can be found). You'll end in a storage, move your character up and you can find a girl with pink twintail being surrounded by guards. 

Save her and you can find the wizard (sorry, forgot his name) in broken fort at amagal. Keep doing the main story until the main story ask you to either save the king of amagal or go to minsk (or dorgania, sorry if i'm wrong). Before going to minsk you should talk to the trader in aldlyn (the one you always see whenever you try to leave your castle through the front). He'll make you the letter which you could use to enter minsk.

To find lucilla, you need to go to the right (this is in minsk okay? Remember it) the guard there will ask you the permission just like the first guard on the entrance did. To get the permission you'll need to go to the official house (the one with a green roof, two red window and a banner) to get it. Go back to the guard and give it to him.

After you go in, walk to the bottom right of the area. You could see a wooden house (it's the only building in that bottom part so you couldn't miss it). Enter it and you could see a character with wings inside. That's lucilla, talk to her and ypu could continue your main story line.

If you want to know how to walk to dorgania permanently, you could give me a reply in this comment.

Cya