Cannot add or delete approvers in the approver's management screen (RSSP)

Summary:

On the SSP screen, purchase requisition users can add or remove approvers from the Approver Management screen when creating a purchase requisition, but on the RSSP screen, there are the following problems.
-Add approver button exists, but it is not clickable
-Delete button does not exist

Why is this difference occurring?

Attached is a document comparing the SSP and RSSP screens.