If the act of approving or rejecting the request is simply the click of a button, then obviously there could be two buttons labeled "Approve" and "Reject" next to the item, but this is clearly a design consideration and nothing to do with the use of English.
If the design is to take the approver (there is a hint already) to a separate screen where the act of approving or rejecting the item must be carried out in more detail, then I would also go with "Approval" as the button label to represent the act of approving (which may still result in rejection).
You generally say that something is sent for "approval" and not "rejection". However, that does not mean it will definitely be approved. It can still be rejected. Similarly, the person doing the approval can be termed an approver, but is never termed a rejecter.
Therefore I maintain that "Approval" as a noun is a good collective term describing the act of approval, which may result in something either being approved or rejected.
From working with enterprise systems, where this query originates, my experience is that the term "approval", as a matter of common usage, is generally deemed and understood to be such an act which may result in either approval or rejection.