Stripe checkout is not enabled
Checkout selected Stripe while checkout is off or the client cannot be created (503 short form Stripe checkout is not enabled.).
Why this happens
Enable flag false and/or secret key missing. Longer sibling tells admins to add STRIPE_SECRET_KEY or save the key and enable checkout.
Diagnose and fix
1. Admin → Settings → Payment Settings — paste Stripe secret key; enable Stripe checkout; save.
2. Or set STRIPE_SECRET_KEY in the environment and restart/recreate as needed.
3. Retry the purchase; offer PayPal/other gateways if Stripe is not intentional.
4. See the longer “not enabled — add STRIPE_SECRET_KEY…” article.