One workaround is to try this sidebar calculator extension
Another workaround is to add any page you want that has a calculator to the sidebar. Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 horizontal dots at the bottom of the sidebar, scroll down to "Custom site panels" and add the URL to the page. You could add https://www.google.com/search?q=calculator to the sidebar for example to always get the Google calculator.