Error "Photos cannot play this video because the resource is unavailable" how do I fix this?

Hi there janejanejanejane, 

It's great to have you in the Apple Support Community. We understand you can’t play the videos you recently imported from your iPhone in the Photos app on your Mac. We can help.

Although the steps in this article were created for an older macOS version than the one you’re using, we recommend that you give the steps a try and keep us posted: If you can't play videos in Photos on your Mac

Cheers! 

I'm guessing you are not using iCloud Photos?

You are likely getting this message because the video has not been copied into the Photos library folder when you imported the video.

If you open preferences within Photos, is the box ticked that says 'Copy items into library'?

Although the thumbnail is showing in the library, it does not necessarily mean that the actual underlying video file is in the library. Hence the importance of making sure the copy items box is ticked when importing to make sure the file is copied.

How did you import the file into the library, was it via iCloud photos or direct import from phone?