Better TweetDeck is provided 'as-is', will not work on the new TweetDeck UI and will not receive any new updates as of February 2023. You can see the full context about that decision in this GitHub issue
Better TweetDeck was available on https://addons.opera.com, but the Opera add-ons team's review times have been very complicated to deal with, so the Opera version always was days if not weeks late compared to the Chrome/Firefox one.
Fortunately, there's a "better" way. It's a workaround, it's not ideal, but at least you'll get updates at the same time as Chrome and Firefox users!
Follow those steps:
Feek free to reach out on Twitter @BetterTDeck if you run into any issues.