I’m trying to log into the app using my user name (I’m old school, in fact classic). This account goes back to when Runescape was first made. But, it doesn’t let me log in. It suggests I log in via my email, still, doesn’t let me log in? I’ve logged into old school many of times over the past years, never had an issue before mobile got released, as just gave up.
Are you using wi-fi or your mobile data to access your account?
Try switching.
Your login is permanent and doesn't ever change. If the error message is suggesting you use your email to login and you know you normally use a 'username', then ignore the error message.