After creating requisition click on approve button.
There you can see The Note field which has Box. there you can enter.
Hope this will help you...
Is it on base purchasing or iProcurement?
If it purchasing forms-- there is note available on the approval screen (before submit for approval) where after click on approve option.
if it in iProcurement - The justification column on the approval selection screen.
Go to Add/Update Requisitions > Add a New Value: Business Unit, Req ID
Click on Add Comments / Edit Comments > Click "Approval Justification.
Secondly, please make sure that "Approval Framework Control" -> "Workflow Approval Required" is turned on for that BU.