I am so sorry to not be able to help you more from so far away. Without seeing the stroller, the error with the folding, I am not sure how to help you. If you studied the manual, as well as a video and the stroller, still doesn't fold, it is possible there is some kind of internal damage - something broken or malfunctioning.
My personal suggestion would be to call/email/visit the seller you bought your stroller at, and ask them for help. They should be able to either show you what's wrong or, if something's damaged, to recommend a spare part they can order or the repair service they would suggest.
If you can, try to make a video of the problem, of you, folding your stroller, and send it to me here - a link for a YouTube video would do... or better yet, send it to the seller. I am not a clairvoyant so I can't know without seeing, understanding your problem better. Nobody will be able to.