Ahh, I think I see the problem.
If I’m not mistaken, here’s the scenario:
You did a quest, you’re nearly finished, but decided to do something else first.
You want to use Co-Op mode, but find that you can’t. Essentially, the Quest has priority, and Co-Op can only activate once the quest is done.
At least, that’s how I think it is. But just to check, is it a main quest (For A Tomorrow Without Tears) or side quest (Monstadt Gastronomy, Sword Cemetery, etc.)?