I had the same problem. Trying to use different approaches to solving the problem ultimately had the same result ... I could not create an Azure subscription when registering in my O365 account.
Work with Microsoft support. The approach that worked for me successfully was to open an InPrivate browser session. Go to https://account.azure.com/ , which causes a credential call in which you must use O365 credentials. Ultimately, a successful result.
BTW> I could only participate in MS support by sending a support request. MSFT reacted to being contacted within 60 minutes of the proposed resolution.
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