Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to "Custom Site Panels" and add the URL for the web version of Google messages. I guess that's https://messages.google.com/web/, but you can double-check.

Open the URL in a tab and log into and pair your phone etc. first if needed. Then click the reload button on the Google Messages sidebar panel that you added.