QuickTime Player can't open/play .mp4 file

Hi,

I have created an export from imovie. The video and audio both display in imovie, but once exported QuickTime player won't allow me to open/view/play the file (as below). I have tried on both my macbook pro and mac desktop. When i click the message 'tell me more' it seems to state it is out of date. But i have no idea how to fix this. Can i play the file?

Please help!

Thanks,

Sam.