The most common issue is with video codecs. We support MP4 videos that are rendered with the common H.264 codec. If you are attempting to upload videos rendered with another codec (i.e. ProRes4444) you will see an error.
Solution:
In this case you can simply re-render your videos with the H.264 codec using your video editor or - in case you didn't create the video originally - use a free application called Handbrake to re-encode them.
Let us know if that works!