How to upload .mov files to iphoto

Here is a previous thread of the subject.

https://discussions.apple.com/thread/8599698

Please report back per Tony's comment. Another solution would be to use Handbrake (freeware but safe to use) to convert the files, this would also flag up any potentially corrupt files. The software allows batch processing from memory so converting 800 files shouldn't be TOO painful.

Is your Mac really running OSX 10.11 as shown below your question? That would be five years out of date by now. Or did you upgrade to a newer system version?

Also, you are mentioning iPhoto, but iPhoto cannot sync with iCloud Photos, and it looks like you were using Photos for Mac and not iPhoto at all.

I am asking, because the error message might be misleading and the problem could be something different.

On a newer system version your Documents folder might be syncing with iCloud, and then you will get the error message every time you are trying to import a video that has not yet been downloaded completely from iCloud. You will get the same error message, if your videos are stored on an external volume with a case-sensitive file system format, or on a volume that is not formatted MacOS Extended (Journaled) or APFS or has been used for Time Machine backups. Where are your videos stored? On your system volume? In your Documents folder? In your iMovie Library? On an external volume?

Download the freeware app video lan. Can that play it. If not the files are almost certainly damaged/corrupted. If it can, while they are playing, open the media information window. (window>media information). What does it say the codec is for the stream of type video?