For someone to be able to approve or reject submitted expenses, they need the Approver permission. This permission will activate the Review tab under the Expenses Report folder.
The account holder or user with the Manage People permission, can configure expenses permissions from the People tab.
📎NOTE: When the Approver permission is on in the Permissions tab, it automatically activates the user email as "expense approvers" under the Settings tab. Once this option is active, submitters can select any available approvers when submitting a report for approval.