Hello mellinda,
Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to play video taken on your iPhone in the Photos app on your Mac. This could be related to the format video is taken on your iPhone, so make sure that both your iPhone and Mac software is up to date. Then change the format the video is taken on your iPhone, and test your issue again.
iOS 11 and macOS High Sierra include support for these new, industry-standard media formats:
- HEIF (High Efficiency Image File Format) for photos
- HEVC (High Efficiency Video Coding), also known as H.265, for videos
HEIF and HEVC offer better compression than JPEG and H.264, so they use less storage space on your devices and iCloud Photo Library, while preserving the same visual quality.
To fully view, edit, or duplicate HEIF and HEVC media on your device, upgrade to the latest version of iOS 11 or macOS High Sierra.
Though capturing in HEIF and HEVC format is recommended, you can capture media using older formats that are more broadly compatible with other operating systems and devices:
- In iOS 11, go to Settings > Camera.
- Tap Formats.
- Tap Most Compatible. This setting is available only on devices that can capture media in HEIF or HEVC format.
- All new photos and videos will now use JPEG or H.264 format. To return to using the space-saving HEIF and HEVC formats, choose High Efficiency.
Using HEIF or HEVC media on Apple devices
Cheers!