If an approver is not able to reject the invoice, switch to the view of the desired invoice. As long as the invoice has not been exported yet, you can still influence the approval process via the "Edit" button on the right side of the document data.

Please note that as soon as you make changes to the process, the process will start over again - even if one or more approval stages have already been completed.

However, the previously granted approvals are not lost, as they are documented in the history as well as the audit trail.