To be able to approve or reject submitted expenses, you must be assigned as both an Approver and an Expenses Approver.

When you configure someone's expenses permissions, select them to be an 'Approver'.

This means when you click on the Settings tab, you're listed to select as an Expenses Approver under Company-Level Settings,
​
​

Once you're selected as an Expense Approver someone can choose you as the person to approve the expense report. You also have access to approve or reject the submitted expense report.