Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
A three month voucher to Microsoft 365 does not sound correct, Microsoft only provide either a monthly or yearly subscription to Microsoft 365.
1
When you purchased the subscription, were you provided with a Product Key, and if so, did you register that on your account on this link?
2
Log into your Microsoft account on this link, do you find an active Microsoft 365 subscription listed there, and if so, what is indicated as the expiry date on the subscription.