You're missing the correct graphics files in the correct directory for the track.
I don't have the particular track that you're referring to at hand, so I'll just give you some general guidelines and suggestions when dealing with GP3 addon tracks.
1.Addon tracks are almost always based on earlier (either original game or addon tracks).
2.Addon tracks use at least some graphics files from the track that they are based on, so you usually need to have the track that the new addon track is based on already installed previously in order for all of the graphics files for the new addon track to be in the game.
3.Many addon track makers often neglect to provide information as to what track their new track is based on and if it is necessary to have this earlier track already installed in order for their track to work properly with all of its graphics.
4.I am going to guess that Patrick F1's Monaco track is probably based on either the original GP2-2k Monaco track or on Bruno Gourdo's addon Monaco track. If that is so, then you will need to install one or both of those tracks into your game. Bruno Gourdo's Monaco track should be in the Old Downloads section of this website. The graphics (jam) files for the original GP3-2k Monaco track can likely be found on "abcdefg"'s website, as they are necessary for his versions of earlier seasons' Monaco circuits.