Photos cannot play this video because the resource is unavailable.

Some video formats are containers, where the video itself is stored separately inside the container or outside the container. You may begetting the error message for three different reasons:

1. if you copied a video from another device or disk, but only the container, without the referenced video resource. In that case you need to to connect the old drive or disk, so photos can access the missing video component.
2. the file system format of the volume where the video is stored, may be incompatible with Photos, for example, if the volume has a case-sensitive format, Photos may not be a le to find the resources inside.
3. Old videos may be using a video codec, that the current version of Photos can no longer handle. In that case use a video converter to convert the videos to a compatible format..