Corrupted Factory Reset

Wondering if anyone can help me, I tried to factory reset my 2012 macbook pro however I did not back up any of my data via icloud/time machine.

I have been able to reinstall mac OS Mountain lion via the key command cmd+R.

Now I am trying to reinstall another os but i cannot find any links/download points for a new OS and I cannot install mac OS Catalina via the app store as it says that I need to upgrade my OS before re installing.

Any suggestions on what might be the problem/ where I can get a new OS from?