Unable to find ATTRIBUTE_NUMBER DFF in payables invoice header DFF

Hello All,

Based upon the business requirement, we are using one data type as a number format: DFF, ATTRIBUTE_NUMBER1 dff in the Payables Invoice Header. If the DFF is blank, the system needs to reject the AP invoice. 

But we are unable to see ATTRIBUTE_NUMBER DFF in BPM—Invoice Approvals, whereas we are able to see and select invoice header descriptive flex field attributes 1 to 15.

Please advise how this can be achieved.