Why can't you do these things? What happens? If there is an error message, please quote the exact complete unedited word-for-word error message.
Please update Lightroom to 7.1 version and check if that helps to fix this issue.
Also, are you getting any specific error message while playing any video files in Lightroom? If yes, please provide the same.
Unfortunately, it still does not work, then go back to Lightroom cc 2015. So far, it works. Unfortunately, we do have an older mac, but I think the new software should also work here.
No errors he just did not play.