When you are trying to redeem a voucher code on an order, the error "We were unable to look up the details of this voucher" might pop up in certain scenarios...
This will mean one of two things, either...
This voucher code doesn't exist in the system i.e. you might have entered the voucher code incorrectly OR
The order where you purchased the voucher hasn't been checked out yet
Voucher codes will only be able to be redeemed once the order they were purchased on has been successfully checked out.