Hello @HollyChristy-8039 ,
Could you please give more details - Are you trying to add members from your organization (user@yourdomain.com) or from outside your organization? If you are trying to add users outside (Guests), the error message is expected. In that case, you can follow my blog below which shows a short video on how to add guests in your Teams
add-external-contacts-in-microsoft-teams
Please mark as "Accept the answer" if the answer helps you. Your suggestion will help others also !
Regards,
Manu