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 platform directly and user management will now be handled through your identity provider.  Users are added to the platform by first providing them with the proper role within your SSO identity provider (i.e. Okta) and then having them log into the platform.

For more information on Single Sign-On, see:  Managing Single Sign-On