This question has come up before and the advice from Microsoft was
"As a long as the tenant has been active for at least a day and you have a commercial version of Office 365 it should work. If you have had the tenant for a while then please private message me your tenant name or use the feedback button in the bottom right of any Secure Score page and we will take a look."
Might be worth using the feedback option in Secure Score though you may get an official response here.
