A save game would make it easier to see what the problem might be.
If you are on your liege's council, and he has council law War Declaration set to Council, you will be unable to create or join any factions unless the council is discontent. If the law is set to Ruler however, you should be free to create or join any factions.
If you are on your liege's council, and he has council law War Declaration set to Council, you will be unable to create or join any factions unless the council is discontent. If the law is set to Ruler however, you should be free to create or join any factions.