Yes, you can. When uploading, video is best handled in H.264 encoded MP4, and a resolution of 720p or higher is preferred.
If the upload stops while uploading, it's likely a connection error/drop, and is not related to the format of the video. Keep trying and see if it keeps happening. If so, please contact firstname.lastname@example.org
We always encourage you to add as much content to your portfolio as possible. Also, in order to appeal to the clients on our site, we also recommend that you add a personal video about you as an artist, what inspires you, and your working process. You can read more in our blog Soply's Latest Tips to Get Hired as a Freelancer