This message may occur when the software has recognized a payment reference that includes letters, or when a UBL invoice is processed that contains letters in the payment reference. AFAS only supports a payment reference that contains numbers.
Solution:
Open the booking.
Click the settings icon at the top of the booking screen (gear icon in the top right) to adjust the booking screen settings.
In the window that appears you can enable the field 'Payment reference' at header level (only applicable if this field is not already enabled). Save the change.
Remove the recognized or entered payment reference in the booking and click 'Save'.
Go to 'To export / Errors' and click 'Re-export' or 'Re-export all'.