after an update my mac won't play you tube video

Reboot through Safe Mode, and try again. That rebuilds caches, and—while you’re booted in Safe Mode—testing with Safari while booted that way also (temporarily) disables various add-on apps.

If that doesn’t clear this, one of the usual causes of these weirdnesses are add-on apps. Add-on anti-malware, add-on ad-blockers, add-on anti-virus, add-on VPN apps, add-on cleaner apps, that sort of thing.

Please download and run and post the output of EtreCheck. Then open a new reply here and then press the button that looks like a printed page to get a text input box large enough to paste the hardware and software configuration report here.

have you tried another browser?