You're already the management account of a different organization. If you try to link your account to a management account, but you've already created an organization with your account as the management account, then you receive an error. To join an organization as a member account, you must convert your account from a management account to a standalone account. For more information, see remove all member accounts from your organization and delete the organization.
Your account isn't verified and activated. Be sure that you fully complete the account sign-up and activation process. Some services can take up to 48 hours to fully activate after the account sign-up process is completed. For more information, see How do I create and activate a new AWS account?
You didn't accept the invitation to the organization, or the organization didn't send you one. Be sure to accept the invitation in the Organizations console. You might need to wait a few minutes before the Organizations console or Billing and Cost Management console reflect that you're a member of the organization. If you didn't receive an invitation, ask the administrator of the management account to send you a new invitation.