You're not on Teams yet, but you can set it up for your organization. [Microsoft Teams Login Error]

If you try to login to Microsoft Teams application on desktop/mobile for the first time using your personal Hotmail/Outllok/Live email account and get the above message after you login then you must know Microsoft is not for personal use. You must be associated with a organization to get started, or sign-up your organization.

1. Click on "Sign up for Teams"
2. You will be directed to a webpage where you would see "Get your organization started today with the free version of Teams"
3. Click on "Sign up for free", you will be asked to login again.
4. You would see 3 options to choose from:

   How do you want to use Teams?
   
   a. For school
   To connect students and faculty for courses and projects, in a classroom or online
   
   b. For friends and family
   For everyday life, to make audio or video calls
   
   c. For work and organizations
   To work with teammates wherever they are

5. If you select option "For friends and family" you will be asked to use Skype.

   Connect with friends and family
   Teams is available on mobile. 
   You’ll need the most recent version of the mobile app. 
   Visit your app store to update or download, then add a personal account.
   If you are looking for audio and video calling from your desktop, 
   Skype is the best option.

6. If you belong to a organization you can set up free teams account by checking "For work and organizations" option.

Difference between Free and Paid Microsoft teams account

Free Teams can have maximum 500k users per compared to unlimited if you have an enterprise license. Even the file storage difference is 2GB/user v/s 1TB/user. Phone calls and audio conferencing is not available in free account.

Check more: https://support.microsoft.com/en-us/office/differences-between-microsoft-teams-and-microsoft-teams-free-0b69cf39-eb52-49af-b255-60d46fdf8a9c