There is a possibility that your macOS or browser version is outdated. If that is the case, you are trying to play a high-resolution video in the VP9 format in Safari, which only supports h.65 videos. There can be times when faulty network connections cause online videos to not play on MacBook Pro. The following article will provide you with several solutions for troubleshooting video playback problems on Mac, including online and offline error codes. Learn more here: https://www.soldrit.com/blog/mac-tip-macbook-videos-wont-play-heres-is-the-fix/