What version of Windows and what version of Opera?
What does https://html5test.com/ say for h.264 (video) and aac (audio) support?
What does https://bitmovin.com/demos/drm say below the video for MSE and EME?
If you goto the URL opera://flags/#disable-accelerated-video-decode, disable the flag and restart Opera, do things work then? If not, if you goto the URL opera://settings/system, disable hardware acceleration and restart Opera, do things work then?
Do you have an "N" edition of Windows? If so, you may need to install (or uninstall and resinstall) the Windows Media Feature Pack.
You might have to download this tweak tool and reset the Windows Media Foundation.
Does Vivaldi have the same problem?
If you goto the URL opera://components and look at the Widevine component, does is say some version for it or does it say 0.0.0.0? Either way, you can goto the URL opera://about and take note of the "profile" path. Then, close Opera and delete the WidevineCDM folder in the profile folder. Then, start Opera, goto opera://components and click "update" for the Widevine update, and make sure it updates from 0.0.0.0 to some version.
If you goto the URL opera://media-internals after trying to play a video, you can click on the section for the video player in question to expand it. There, you can see what errors (if any) there are. You can also see what type of video and audio Opera is trying to play, which may point you in the right direction of the problem.
Sometimes sites block Opera. You can make a site think you're using Chrome instead of Opera. Install this user-agent extension and select Chrome for Windows for the site you want to try it out on. Then, click the edit button at the bottom of the extension button's dialog to edit the string it uses for Chrome for Windows. You want it to be what Chrome says for the user agent at the URL chrome://version. Right now that's Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.122 Safari/537.36.