Trying to sign up for Free Teams

I am trying to setup Teams for a customer of ours (.@custname1.com) but it seems that I cannot for some reason.

When I do it gives me the error that " hasn't been added to your organization's directory. Or contact Admin or try a different email""

Now I had done this this way for a 3rd customer(.@custname2.com) and it went OK.

I did find out that (.@custname1.com) is a guest in someone's else's Teams.

But the customer of custname1.com wants Teams themselves so they can use it as well. Would that be the reason why I cannot set it up for them?