A voucher is NOT A TICKET.
Whether you bought a voucher pack or were given a voucher by someone else, you will need to redeem that voucher for an event ticket. Think of it like a flight voucher—you still have to book your flight if you want to go somewhere.
To redeem your voucher, add the corresponding event ticket to your cart, add the voucher code to the coupon/voucher code field and apply it, then check out as usual. The cost of a single ticket will then be $0 and you will be registered for the event.