Can't talk to Tullius about Season Unending

I can get Ulfric to talk and agree to join in the peace treaty but Tullius will NOT even give me the option.

I have NOT joined either side in the civil war. However, I have completed the starting quest for both sides; yet, I still have not taken the oath for either side.

What can I do to help fix this?

You need to actually join a side in order to do this quest. Finish one of the two "join faction" quests to the point where the other one fails and you swear the oath. Then continue seasons unending.

I strongly recommend you don't though, it is absolutely atrocious and made me rage quit skyrim for a year or so. When I returned to skyrim I avoided this quest by joining a side and winning. You are also missing out on a bunch of achievements and a lot of interesting missions by not doing the civil war properly.

If you do choose to proceed with this quest, consult this section http://www.uesp.net/wiki/Skyrim:Season_Unending#Notes which lists the various quests and achievements that can be broken by this quest, and how to avoid these problems.

I was able to fix this by going into the console and resetting the quest

resetquest CW01A

After that I was able to tell Tullius about the peace talks.

Make sure you keep a save from before doing this as messing with quests in the console can break things.

You must be during the quest when you have to kill all the bandits at a fort in order to join the Legion. When you talk to the General without killing the bandits, he says:

• I believe the Legate has gave you a contract, and must be fulfilled. Dissmissed.

You need to kill all the bandits at the fort and then talk to the Legate and the General.