Can't open this video 🎞️

It's recorded in HEVC / H.265 and this browser has no HEVC decoder at all — so it can't play it and can't convert it either (converting still requires decoding it first).

What works (easiest first):

• Upload from your phone. Open this page on the phone: phones decode HEVC natively, and there the app converts by itself.
• Stop it at the source: iPhone → Settings ▸ Camera ▸ Formats ▸ "Most Compatible". Samsung/Android → Camera ▸ Settings ▸ Advanced recording options ▸ turn off "High-efficiency videos".
• Convert on this computer and upload the result. With ffmpeg it takes seconds for a long clip:
  ffmpeg -i video.mp4 -c:v h264_qsv -global_quality 24 -c:a copy out.mp4
  (use -c:v libx264 -crf 23 instead if you have no Intel graphics)

Google Photos hands the file back in HEVC, which is why a video that plays fine on your phone fails on a PC.