These type of questions require some info up front.
What version of Opera?
Is it regular Opera or Opera GX?
What version of Windows?
Windows 10 Home 64 bit version 20H2
Is that version an "N" edition of Windows? If so, you might need to install the Windows Media Feature Pack for your OS.
At https://html5test.com/, does it say "Yes" for h.264 support and "Yes" for AAC support?
At https://bitmovin.com/demos/drm, does it say "Detected, using Widevine" above the video and does the video play? For MSE below the video, does it have "video/mp4; codecs="avc1.42c00d" checked? In the EME section, does is "Widevine" checked?
Did you try installing https://addons.opera.com/en/extensions/details/user-agent-switcher-5/ and setting the site to mask as "Chrome for Windows" (including editing the "Chrome for Windows" string to be Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36)?
Did you try going to the URL opera://flags/#disable-accelerated-video-decode, disabling the flag and restarting Opera?
Did you try going to the URL opera://settings/system, disabling hardware acceleration and restarting Opera?
None of these have worked for me and I'm not sure what to do.