Short answer: [400] [133010] (#133010) Account not registered means the phone number is known to Meta but has not been registered for the WhatsApp Cloud API, so it can't send yet. In Make, add WhatsApp Business Cloud › Register a Sender, pick the same connection and phone number, enter a 6-digit PIN, run it once, then delete the module and retry.
Everything here works on Make's Free plan. The steps come from Make's WhatsApp Business Cloud troubleshooting page and Meta's Cloud API docs; we did not register a number ourselves for this guide.