NOWPayments API key is not configured
nowpayments-api threw because the API key is empty before calling NOWPayments.
Why this happens
Checkout toggle may be on without pasting the key. Sibling of broader “checkout is not configured/enabled” messages.
Diagnose and fix
1. Admin → Settings → Payment → NOWPayments — paste API key; save.
2. Enable NOWPayments checkout; set IPN/webhook secret for paid callbacks.
3. Use sandbox keys only with sandbox/test mode if offered.
4. See NOWPayments Help article.