Someone has already set up Teams for your organisation Ask your admin to get you started with Teams,

Is you are signing up for a free teams account and you use a registered domain that is already somewhere within Microsofts ecosystem (I.e. Office 365, a trial account previously) then it will have the message above. The only resolution to that is to contact Microsoft and ask them a.) where it is and b.) to remove it. There are many reasons it could have ended up there including another organisation adding it to their account. We typically recommend outlook because you can use those email addresses to sign up to Free accounts. Gmail addresses work too. If you have your own on your own domain and you are getting that message you need to contact support

Best, Chris