I'm sorry. You want to help me but you keep offering solutions to the iPhone when I emphasize that my problem is on the computer and not on the iPhone
But your iPhone may be creating the problem for your computer, if you are still using iPhoto on your computer. You said you took the videos with the iPhone, didn't you?
iPhoto cannot play any of my videos I am taking on my iPhone with iOS 11, while Photos for Mac can. You simply have to set the iPhone to take the videos and photos in a format that iPhoto can read and import. The last version of iPhoto is more than two years old and cannot deal with the new video formats, and there is no hope that it will be updated to be compatible with iOS 11 video formats.