Sure, you can have an opinion on something. Something like, Green is the best color, or, Chocolate tastes better than vanilla. Those are completely subjective points with no possible right answer. No matter how many people agree or disagree with you on the perfect level of spiciness when it comes to chili, you won’t be either right or wrong; you’ll just have your opinion.
You can also have an opinion on something that’s practically impossible to know, like if World War One would still have happened if Franz Ferdinand had not been assassinated. Obviously history would have been different, but to what extent? Sure, there were lots of indicators that Europe at that point was just waiting for an excuse to erupt into war, but there were also a myriad of unpredictable variables involved, and we can never really know for sure (see Chaos Theory). So you can have all the opinions you want on the matter.
But you can’t have an opinion on a provable subject. For example, you can’t have an opinion on how old the planet is (“400 years, but that’s just my opinion”) when evidence to the contrary exists (especially when evidence is well-established- in this case, in many fields).
That’s not “having an opinion”. That’s rejecting reality. And disregarding evidence and actively undermining people who point out the way things really are- that’s not proving you’re “right”. That’s not having a reasonable discussion. That’s *gaslighting*.
You don’t get to “have an opinion” about my lived experience. You don’t get to define my reality or silence my voice. And you sure don’t get to act all innocent while attempting to do so.