Some iPhones record with the video codec HEVC/H.265. This codec can’t be processed by Unscreen. Here’s a few ways to deal with this:
You can use an online conversion service like Zamzar to convert your video to MP4, which should work on Unscreen.
You can change the recording settings of your camera and change it to a more compatible format, e.g. AVC/H.264.
You can use Safari on your iPhone or Mac to upload to Unscreen. In the process, Safari converts the video to a compatible codec.