If recruiters decide to manually reject the offer on behalf of the candidate or rescind the offer, they must select an option in the Rejection Reason drop-down list of the Change Status of Offer dialog box. Moreover, depending on the rejection reason they select, it might also be mandatory for them to type a value in the Note field of the dialog box.
Rejection reasons aren't preconfigured. You need to configure at least one rejection reason so that recruiters can decline or rescind the offer; otherwise the Rejection Reason drop-down list appears blank. See Configure Candidate Decline and Offer Rejection Reasons.