cant play videos in photo library

Hi jacalyn272,

Thanks for coming to Apple Support Communities for help today.

We noticed your Mac is running 10.14 and we have an article containing steps that may resolve the issue for you: If you can't play videos in Photos on your Mac.

"To prepare for future versions of macOS, find and convert incompatible media in your Photos library.

When you access older video files in Photos on macOS Mojave (macOS 10.14), you may see an error message: "Cannot Play Video. Photos cannot play this video. Do you want to try opening it in QuickTime Player?"

Follow steps in this article to convert the video file. If you don’t convert your files ahead of time, they won’t be convertible or usable in future versions of macOS.

Convert media files in QuickTime Player

1. Open the Album section in the Photos sidebar.
2. Click Videos in the Media Types album.
3. Click a video in the album to play it. If the file is incompatible and you need to convert it, a message asks you to open the file in QuickTime Player. 
4. Click OK.
5. Wait while Quicktime converts the file. 
6. Click Save, which saves the file to your desktop.
7. Click Replace, when asked. This replaces the incompatible file.
8. Find the new file on your desktop and drag it into your Photos window."

If that doesn't resolve the issue, let us know more details.

1. Do these videos play on iCloud.com through a browser if you use iCloud Photos?
2. Do new videos added play?
3. What was the source of these videos?

Have a fantastic day!