You have to install the Roboform program. Then, you have to install the Roboform extension in Opera that communicates with that external program (using Native Messaging). Then, you'll want to disable "Offer to save passwords" at the URL opera://password-manager/settings.

However, things still might not work in Opera because the Roboform program's Native Messaging part has to allow Opera to communicate with it it. It might only allow Chrome, Firefox and Edge for example.

So, assuming you have everything installed right, it's probably Roboform not allowing Opera where you have to report it to https://help.roboform.com/hc/en-us/.

Now, https://help.roboform.com/hc/en-us/articles/360015915412-How-to-add-the-RoboForm-extension-to-your-browser-s#Opera implies Opera is supported, but maybe that support is broken. But, for good measure you can test with Opera GX, Opera Beta and Opera Developer (you can download them from their sections at https://www.opera.com/download) to see if it's just a bug in Opera Stable.