The mandatory field forma pago is missing in the eInvoice XML file and you expected that the XML file contained the field FormaPago with appropriate external values.
SAP S/4HANA Cloud
This is not a mandatory tag for eInvoice scenarios according to documentation provided by SAT.
This is not an issue/error in the standard eDocument Solution and to populate the <FormaPago> tag for eInvoice scenario, you should adjust your current scenario, by fulfilling the field "Payment Method" and the value mapping "CFDI_PAYMNT_MEANS", or by implementing your own logic in the app "CUSTOM LOGIC" and add the BAdI "Mexico: Filling of Payment Method for Payments".
Please, revise and follow directions in SAP Help Portal APP Extensibility: eDocument Cockpit:
Mexico: Filling of Payment Method for Payments allows you to define your own logic for filling the Payment Method field when eDocuments are generated for electronic payment receipt complements.
FormaPago, eInvoice, Payment Method, value mapping, CUSTOM LOGIC, eDocument Cockpit , KBA , CA-GTF-CSC-EDO-MX , Document Compliance Mexico , How To