How to open new iPhone videos from latest OS in old FCPX 10.1.3 in Mavericks.

Hi folks

I'm sure there's a simple answer to this.

I'm running two Mac Book Pros, one with Catalina and one with Mavericks 10.9.5. I've got an iPhone 7 with the latest OS. Both FCPX 10.1.3 and Quicktime in Mavericks don't accept the videos from the iPhone 7. These are 1080p HD videos.

I've manged to convert these to work by using VLC, but is there a specific reason why the files aren't compatible? For example, if i try and encode one of the videos in Mavericks, i get this error as part of a long message:

<<<< RFCruncherThreadWrapper >>>> RFCruncherThreadProc: RFCruncherThreadProc: RFMediaCruncherTask of VideoFrameRateConverter has returned an error: -12906 (thread:0x10b6cd000).