ThisOldGuy When you launch the app, it has that welcome screen and asks for you to type your home instance's domain, right? In that box type mastodon.social. You don't need to add anything else. The app finds that instance and it pops up under that box and you can tap on the entry there and it'll open a browser for you to log in.
Unsure why it would say the server doesn't exist. Maybe your keyboard autocorrected or added an unnecessary space? Make sure there are no spaces, and make sure not to include the @ since that's just a naming thing on Mastodon, not part of the domain name.