This is Ciara from the Content API support team. Thank you for reaching out.
According to our documentation, after the creation of a new account it may take a few minutes before it's fully operational. Kindly wait for a few minutes before making requests for the newly created account. You can use the accounts.list, that is used to lists the sub-accounts in your Merchant Center account to check if the new account appears on the list. If you still encountered the User cannot access account <account_number> error, make sure to check that you're targeting the right account or register yourself as a user of the account in Merchant Center (Settings > Users).
Let me know if you have any questions.