I recently got back from a trip to France where I created some videos with my Samsung S9 and also a GoPro Hero 6 Black.
All photos could be imported fine into my Lightroom CC Classic catalog but I noticed that the importer reported errors for some Galaxy S9 videos. I'm pretty sure some other videos of the phone with the same settings (HEVC, FPS, resolution) could be imported just fine. Funny thing is, that after I renamed the extension to AVI they could be imported straight away - also the import dialog showed a thumbnail for those videos, which it didn't do for those videos having their original "mp4" extension.
Unfortunately later the day I also tried to import my GoPro videos. The timelapse videos could be imported without an issues but all other videos couldn't, and the import dialog showed no thumbnail for those videos again. It failed the import with the following message: The movie files contain no video frames.
Followed by a list of all those videos.
The trick with renaming the extension from "MP4" to "avi" didn't work here, also the thumbnail was still missing in the import dialog.
Of course the files are not corrupt, I can play them without an issue on the same machine using Quicktime or VLC.
I was able to import GoPro videos to Lightroom 2 months ago, and I'm not sure what changed. Well the Lightroom version changed (now CC 7.4) but I think I also activated HEVC on the GoPro and use a higher resolution and more FPS to record the videos (4K and 50 FPS).
I checked the following adobe page to check which video formats are supported but it is not very detailed: Adobe Lightroom supported video file formats
The files have a "MP4" extension (and are in MP4 container), so they should be supported but maybe something of the content/codec is currently not supported:
MP4 Container & Extension
I'm using Lightroom CC 7.4 on the latest version of MacOS High Sierra (installed l latest update yesterday, makes no difference) on a iMac Late 2015, i5 CPU, AMD Radeon 395.
I believe that Lightroom cannot handle the combination of codec and 4K with that FPS settings at the moment but as I am on MacOS I fear that I cannot install an additional codec myself for Lightroom to support those kind of files.
Update: I now tried the following, open such a file in Quicktime Player (on MacOS) and exporting it, using the same resolution (4K) and also HEVC codec (it keeps the 50 FPS) -> on importing such a file Lightroom now says that it cannot import because it doesn't have the codec (which I don't understand, as it was converted with a system component (Quick Time) and could be played on the computer, so the codec is installed). After this failure I tried the export again but unchecking HEVC this time, so it was converted from HEVC to H.264. The import dialog showed the thumbnail and I was able to import the file (also with 4K @ 50 FPS). So it seems to me that the HEVC (version?) GoPro uses cannot be imported into Lightroom??