“You can’t just ‘think that someone is right’ that something is assur. That’s not how halacha works.”
No, you can’t make up a psak, but I don’t think that’s what most people mean when they use that phrase. Most people B”H have learned a fair amount of halacha and if someone asks them something, they will say “yes, I think so” if they don’t remember exactly. That doesn’t mean they made it up; it means that at one point they did learn the sugya and it’s just not so fresh in their minds anymore.