What do I do if ZATCA rejects an invoice?

A rejected invoice was never recorded by ZATCA, as if it was never issued. Open Settings → E-invoices sent and read the reason in the ZATCA notes column; it is usually missing customer data (address or VAT number).

1. Fix the missing data (on the customer's card, for example).
2. Open the invoice, correct it if needed, and press Save: the system sends it again automatically.

Alternatively, cancel it the ordinary way; it needs no credit note because ZATCA never accepted it.