Go step by step and test.
1. Force Quit Safari, launch Safari holding the Shift key down.
How to force an app to quit on your Mac - Apple Support
About pop-up ads and windows in Safari - Apple Support
2. Startup in Safe Mode.
Start up your Mac in safe mode - Apple Support.
Quote:
It also does a basic check of your startup disk, similar to using First Aid in Disk Utility.
And it deletes some system caches, including font caches and
the kernel cache, which are automatically created again as needed.
Try playing the video in Safe Mode.
If it plays video, do you have Display Link app installed?
3. Try reset NVRAM if it is a Mac with an intel processor.
Reset NVRAM or PRAM on your Mac - Apple Support