As an admin, you have the ability to approve requests on behalf of others, e.g. if an approver is not available.
Go to Team Expense > Team Expense Claims > All Records
Open the request that you wish to approve.
You'll see a message that states that you can approve/reject the request as an admin. Click on the 'Approve' button.
Once it's approved, the original approver will be notified, and there will be a log that the request has been approved by you.