Make sure the trip customer/employee/stakeholder is trying to request meets the validity period you originally set for the voucher. If the voucher code still isn’t working, check also whether the voucher is in “Active” status and has been “Enabled” - you can check this from the voucher list page. 
 

Customers/employees/stakeholders can not redeem multiple vouchers for 1 campaign. Even if they have multiple voucher codes, they can use only 1 voucher for the same campaign.