Unable to update the Original Receipts Required checkbox in Audit Expense Line

During the Expense audit process, i'm not able to un-check the Original Receipts Required checkbox for expense lines that has missing receipts.  There are cases where employee indicates that receipt is missing and this claim is valid.  In order to audit this expense line successfully, I will diable the Original Receipts Required checkbox and complete audit.  This is what users are doing in our current R11i instance.  However, in the new R12 test instance, i'm not able to update/unc-check the Original Receipts Required checkbox.  Message shows that Read Only Checkbox Checked.

Appreciate your advice on how to update the Original Receipts Required to Not Required in the Audit page.