Office 365 is a subscription based product. You should have Microsoft account with a valid office 365 subscription to use office 365. If you are unable to activate office 365, login to www.office.com/myaccount and check if you have your office 365 subscription status as active.
You can purchase office 365 product key card from any store. You should scratch the card and retrieve the product key. Once the product key is noted, open www.office.com/setup and enter the 25 character product key.Click next and setup your product. It will activate office 365 program.
If you want to install office 365 later stage or on a different machine, login to Office account and click the install button.
Here is a case where there is an issue with activation when you have valid office account.
Office 365 users can have 1 TB cloud space. You shall store all the important files and folders on this onedrive space. This data is useful when you want to change your computer. The latest office 365 helps you to work directly from the cloud. This makes you to feel like
You can activate office 365 using the valid login credentials through which office product is purchased. You can also share the office 365 plan to the other users in the family or you can install it for yourself on a different computers in the same account. Office 365 program works on all the operation systems like windows and MAC.
To fix Unlicensed Office 365, check this link to solve the error.