What can't I use my video recorded with my iPhone?

The most common reason for this is that some of the iPhone apps record in a newer format (HEVC aka H.265) that isn't compatible with most browsers.

To avoid this issue when you are capturing video with your iPhone:

Open Settings > Camera > Formats.
Under Camera Capture, you have two options: High Efficiency, or Most Compatible. Choose Most Compatible.

If this is for an existing file you have, please try the following workarounds:

Make sure you are using Zubtitle on a desktop or laptop computer (you may need to move your video from your phone to laptop).
If you are using a Mac, try using the Safari browser
If you are on a PC, you'll need to convert the file to an .mp4 or .mov with a H.264 format. Here's an article that discusses a couple ways to do so: http://www.pavtube.com/convert-h265-to-h264.html

If you need additional help, send the Zubtitle team a message via chat.