Should an error message have taken you to this page or you simply cannot sign up it is very likely that your Microsoft Account is not flagged as being an 'Adult'.
What determines if an account is an adult account:
Xbox LIVE determines whether an account is a child account or an adult account based on the age of the account owner. Account owners who are younger than 18 are given child accounts in the United States and the UK. The legal adult age varies by country and region. For example, the adult age in Korea is 20 but in the UK the adult age is 18.
How to check whether your account is flagged as an adult account:
You can check if your account is flagged as a child account or an adult account through your accounts Family Safety settings.
For further support on how to convert an eligible child account to an adult account please contact Microsoft Support.