This may be caused by too much cookies and/or cache. I would recommend clearing those from your browser. If this is not the case, it could be too much device memory, I would recommend checking up on that.
Hello @Senodabossbaby I'm afraid you can't download transparent mp4 video - that mp4 format does not support transparency. There always has to be a background, either a colour, or an image or even another viedo, but there has to be a background.
You may then ask why can we remove backgrounds from videos at all in Adobe Express? Well, so we can layer them over other elements to create interesting compositions. The key is that the finished mp4 that we download always has a background of some kind.