Certain types of organizations aren't eligible for Microsoft Teams free. See below for information.
If you're signed in to an Office 365 work or school account, you won't be able to sign up for Teams free. If you have Azure Active directory and are not licensed for Teams, you may be able to initiate the Microsoft Teams Exploratory experience. Go to Get Teams for free, scroll down (nearly to the bottom of the page) to Get Microsoft Teams for your organization today, and then click Sign up for free to see if you're eligible.
Accounts at schools or other academic institutions aren't eligible for this Teams free offer. However, teachers and students are eligible for a free version of Office 365 that includes a full version of Microsoft Teams, and more. For info, visit Office in education.
Accounts at U.S. government institutions aren't eligible for this Teams free offer, due to their unique requirements. To learn about Office 365 plans for government organizations (including a free trial), see Office 365 Government plans.