There could be a few reasons why your MP4 file is not compatible with CapCut. First, ensure that you have the latest version of CapCut installed. If the issue persists, it could be due to an unsupported codec or specific encoding used in the MP4 file. You can try converting the MP4 file to a different format using a video converter tool, making sure to use a widely supported codec like H.264. Alternatively, you can also try updating your system software or contacting CapCut support for further assistance.