Stripe is not configured — add STRIPE_SECRET_KEY or save the secret key in Admin → Settings → Payment Settings
Admin refund (and mapped codes) need a Stripe secret key under Admin → Settings → Payment Settings (or STRIPE_SECRET_KEY).
Why this happens
Refund/API path requires Stripe credentials even when the payment row exists. Sibling of the shorter Payment Settings wording used on top-up.
Diagnose and fix
1. Save the secret key in Admin → Settings → Payment Settings or set env STRIPE_SECRET_KEY.
2. Retry the admin refund on a completed Stripe payment.
3. Non-Stripe payments still hit Refunds are only implemented for Stripe payments.
4. See Stripe and Only completed payments can be refunded.