@KinmenBoy , this error message can be related to the fact that this user already created a Freemium Webex Teams account in the past.
If you verified your email domain (https://help.webex.com/en-us/nxz79m5/Add-Verify-and-Claim-Domains) you should be able to convert these users and add them to your Org: https://help.webex.com/en-us/nceb8tm/Claim-Users-to-Your-Organization-Convert-Users
