Welcome to the forum. The usual cause of this is misreading the code on the voucher. Make sure you are entering it in the correct section on your My Account > My Passes page
and try a few different combinations, especially where a letter may be a number or vice versa, e.g 5(five) and S, 1 (one) and I (capital i ), 0 (zero) and O (capital o) etc
But if you still can't get your voucher to work, go to
and choose the option for live chat who can check the code for you.