MP4 file support message

Hi

I'm busy building an induction course with pre-recorded video material. I've been editing the videos in Camtasia and uploading them to Storyline 360. Did 11 videos and decided to redo the 1st video. When trying to replace the old video with the new one, I got a message saying that storyline does not support mp4 files and I must choose a compatible file. Does the same if I just import the video file from the folder. Can anyone help please? 

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