Microsoft is no longer offering standalone Outlook.com premium subscriptions to new subscribers. This ability is now available only to Office 365 customers and not to regular Outlook.com users.
Microsoft has made the following statement:
The Outlook.com Premium standalone offering was closed to new subscribers in October 2017. Many of the benefits in the standalone subscription are now included with subscriptions to Office 365 Home and Office 365 Personal.
If you are a user of Outlook.com Premium, you can continue using its features. You can renew your subscriptions and continue receiving the same benefits.
Office 365 subscribers will get all features of Outlook.com except the personalized email address. It won't be available in Office 365 Home or Office 365 Personal subscriptions.
Office 365 subscribers will have a completely ad-free user interface. Banners in the message list won't appear for them. Also, they will get 50 GB for email storage versus only the 15 GB available for a regular Outlook.com account. As part of this move, Microsoft will be increasing storage limits to 50 GB for all Outlook.com users whose mailbox size is 12 GB or larger.
Microsoft is currently rolling out the updated Outlook.com service, so it may take some time before it affects your account.
You might be interested in reading the official support page related to the upcoming changes. You can find it HERE.