.mp4 files recorded on the R5C are perfectly standard MP4 files, encoded as either H.264or H.265, depending on which you chose. I routinely copy these to my PC using a card reader and use them as is, without using XF utilitity or any other Canon software.
However these files are very high bitrate. They will generally not play directly, because your computer likely isn't powerful enough. For example, when I double-click one of these files, the video jerks forwards a couple of times then freezes, leaving just the audio. But when I load the files into a video editor and edit with them, they're prefectly fine.
For more innfo on this, see this thread: https://community.usa.canon.com/t5/EOS-DSLR-Mirrorless-Cameras/Canon-R5-video-file-sizes-trouble-dow...
If you're having some other problem, then you need to provide more information. Saying that some files "cannot be played" is no use. Do you mean that you get an error message? Does the player crash? Does it play jerkily/badly? Does the pcture look wierd? You are giving us no clues at all.
What record modes, exactly, are you using? What exactly are these files called on your computer? Providing some screen shots might help.
I get that this is frustrating, but if you can't tell us what is actually going wrong, then we can't help.
