Stripe is not configured — add STRIPE_SECRET_KEY to the environment or save the Stripe secret key in Admin → Settings → Payment Settings
Checkout session creation refused Stripe because no secret key is available (env or Admin → Settings → Payment Settings wording).
Why this happens
Same root as shorter Stripe is not configured variants — this string is the full “environment or Admin Settings” form used on session checkout.
Diagnose and fix
1. Set STRIPE_SECRET_KEY in the environment or paste the secret key in Admin → Settings → Payment Settings; save.
2. Enable Stripe checkout if members should pay with card/Stripe.
3. Retry checkout; configure webhook secret separately for paid events.
4. See sibling Stripe-not-configured articles from Phase 41.