Accessing Microsoft Teams for Education using school or organizational credentials
Hello mariarosaria
Thank you for participating in the Microsoft Community, I am Skypper G. I am an independent advisor, let me help you with this
That error is due to MS Teams Service is disabled for all users in the organization you are trying to sign in.
You will need to contact with the administrator for that organization and this is the steps they need to do in order to enable MS Teams:
Open https://portal.azure.com with your O365 Tenant Admin account.
Go to : Azure Active Directory
Go to : Enterprise Applications (on the left side bar)
Go to : Microsoft Teams - which will open a Slice for MS Teams settings
Click: Properties
Enable for users to sign-in? : YES