Mar,
Dear Mar,
There are few setting which need to be check in system, Please check basic FI setting for automatic payment(FBZP) then Go to T.code F8BZ and check whether respective setting is done or not? Also check the F8BF and F8BH for consistency check.
Once this customization completed,Please check Table T042I and T042Y with transaction SE16N.
Entries should be same in both the table.
Let me know incase you have any further query.
Regards,
jain