I'm using the Lightroom CC Mobile app on my iPhone7 to move videos from my to my iMac (running High Sierra) in Lightroom Classic CC (7.2). The video is being shot in 1080p at 60fps. When the video is uploaded into Lightroom it does not play and the thumbnail displays a waveform (see screen shot). When I locate the physical video on my mac it plays just fine using quicktime.
The error message indicates Lightroom has encountered problems reading this video.
I've tried recording videos in 720hd at 30fps and 1080p at 30fps and the same issue applies.
However, when I record slo-mo (720p at 240 fps) and use the app to the sync the video from my phone to the Mac those videos display and play just fine.
Most likely the videos are in the new HEVC format, which LR CC supports but LR 7 does not. You can tell your iPhone to record in the compatible MP4 format by checking the option Settings > Camera > Formats > Most Compatible.
Also, please add your constructive opinion to this feature request in the official Adobe feedback forum: Lightroom Classic: Please support HEVC (H.265) video support | Photoshop Family Customer Community . Be sure to click Vote in the upper-right corner. That will make it more likely Adobe will prioritize the feature, and you'll be notified when they implement it.