Hi Emily!
Thanks so much for reaching out. I'm sorry to hear that you ran into this snag when attempting to upload your video. Happy to help!
One troubleshooting step that you can do is to re-encode your video using a tool called Handbrake. Here's a helpful video with instructions on how to use the tool.
If re-encoding your video doesn't work, would you be willing to share a copy of your video file here or in private by opening a support case so we can take a closer look at what's happening? We'll be sure to delete it when we're done testing!
Looking forward to hearing from you!