What to do if I have found a major issue with my model before a conference presentation?
Things like this happen. No one should be ridiculed for honest errors. It's a good thing that you found this one before someone else did.
Your description suggests that some (even much) of your work is still OK, since you say only that the problem  might affect your conclusions.
If you can describe the problem (even with no solution) in advance, you should.
Does the conference format allow you to introduce your talk with some preliminary comments, either via zoom (or some such) or with a short new video upload? Ask the organizers.