NOWPayments is not configured
nowpayments-api refused to create an invoice because NOWPayments is not considered configured (broader check than “API key empty” alone).
Why this happens
Required settings incomplete (API key and/or enable flags). Sibling of NOWPayments API key is not configured and the checkout not-configured/enabled messages.
Diagnose and fix
1. Admin → Settings → Payment → NOWPayments — paste API key; enable checkout; save.
2. Set IPN/webhook secret so paid invoices can credit accounts.
3. Retry crypto checkout; if key is set but create still fails, see invoice-URL / checkout articles.
4. See NOWPayments.