Single Sign-On is enabled for your account which limits your ability to add users directly.
When SSO is enabled, you can no longer invite users from the CloudHealth platform directly and user management will now be handled through your identity provider. Users are added to CloudHealth by first providing them with the proper CloudHealth role within your SSO identity provider (i.e. Okta) and then having them log into the CloudHealth platform.
For more information on Single Sign-On, see: SSO Management