Thank you for your response!
Here is a wetransfer link with the project files - https://we.tl/t-MGeXr4V5Ir
The error message is that after export and I try to watch in quicktime, it says the file may be damaged and can't play. I can watch .mov in MPEG streamclip and VLC but cannot convert it into a different codec for my comp to natively play.
The source files are mp4 & mov.
Export settings, I've done various kinds and they all don't work. I tried changing frame rate - it was shot in 30fps on my camera and I dunno what my samsungs9 records in, one clip is being used from t