Not that it helps you, but you did nothing wrong - the UI is just confusing for 2FA on Lemmy right now. You're supposed to enable it, then press F5, and the field will then display a 2FA code you can add to your authenticator or convert manually into a QR code.
If you leave the page without hitting F5 and getting that code, you're locked out.
Hope the admins sort it out for you.