Sign in to pay
Premium checkout uses premium.checkout.signInToPay as the submit label/path when the buyer is a guest and must authenticate first.
Why this happens
Checkout cannot attach the purchase to an account until sign-in. Distinct from gateway-not-configured errors after you are signed in.
Diagnose and fix
1. Sign in, then complete Pay now.
2. If already signed in and still blocked, refresh and clear a stale guest session.
3. Prefer starting from pricing while signed in (Sign in to buy).
4. See Buy Premium / Premium Pro.