Factory reset MacBook and can't download a new OS and am stuck on OS X Mountain Lion (10.8.5)

I have a MacBook Pro 13" Mid-2012 and I recently factory reset it because I was selling it but I changed my mind because it was my first MacBook. Anyway, I can't open ANY web pages (except for a few such as big companies like Facebook), and I'm trying to download macOS Catalina (the latest OS it can run), and the only way I know how is through How to download macOS - Apple Support but I can't open it because Safari thinks it's a non-Apple website. Is there any way I can download macOS Catalina from the App Store or make it so Safari will open pages even though "Safari can't establish a secure connection to the server"? Also, there isn't a lock anywhere and all there is is a little world icon before the URL. Any help would be much appreciated! (This is on Safari 6 by the way.)