The error message appears due to there is "null" value that cannot be left empty for "Party contact number". To resolve the issue, start by checking each bill document number associated with the error message.
Use the Actions menu (three-dots button) select View Transaction and note down the customer number.
Go to eInvoicing, Company, Contact Details-Contact No: This is required. or,
Go to eInvoicing, Customers, Actions-Menu, Contact Details-Contact No: This is required.
Note: After editing company or customer details, transmit again the invoice and get status.