What status does the expense have? Only expenses that have the status 'submitted' can be approved. These are visible in the 'To do' tab of your approver expense overview.
Are you able to see the 'approve' button? If this is not the case (and you do not have access to the company settings) please contact your company administrator.
Are all required fields filled in? It could be that an expense has been submitted before a certain field has been made required by the administrator of the company account.
By opening up the expense and clicking on the 'edit' button, you will be able to see which field is missing as it is flagged in red. If you are unable to edit an expense, you probably do not have the permission to do so. In this case, please contact your company administrator and/or reject the expense.