Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello, thank you for reaching out to Microsoft Q&A.
Please open an incognito/private browsing window in any web browser and purchase the Microsoft 365 subscription from the following page:
https://www.microsoft.com/en-us/microsoft-365/buy/compare-all-microsoft-365-products
Make sure you sign in with the Microsoft account where you want the subscription to be added.
Then check whether you can complete the purchase successfully.
If not, try using a different browser, device, or network (such as mobile data) and see if you can complete the purchase.