What to do if I have found a major issue with my model before a conference presentation? I submitted an abstract to a major conference in my field. I am presenting tomorrow at the virtual conference and the video is already uploaded, no conference proceedings were submitted. However, today, I came across a major issue with the modeling technique used in my research. This might be seriously challenged tomorrow. Also, this might influence the conclusion of the results as well.
How do I answer if the issues with my modeling strategy are questioned during my presentation? I can't remove my talk at the last moment. I am worried that I will be ridiculed tomorrow.
What can I do?
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.