Are you unable to call 4444 to give the voucher number, @Kellygourlay11?
The recommended ways to use a voucher are to call 4444 free from your mobile, or 08456 062 277 from a landline, enter the number on your receipt and follow the instructions. Top up between £10 and £50. Your voucher will expire after twelve months.
Other ways here: Guide How to Top Up (PAYG)
Or is something else preventing you from keying in the voucher number?
Seems to be a current issue.