Your domain may be in use if someone else signed up with it
Users with a work or school email can sign up for some Microsoft online services without involving their IT department. For example, services like Office 365, Power BI, and Rights Management Services. By completing self-service signup, which they do by providing their work or school email address, they have access to functionality included in the service but they can't administer the service (add users, add licenses, manage the domain, and so on).
To get the ability to administer the environment, an eligible admin can take over control of the account. They do this by verifying that they own the domain that was included in the work or school email addresses people used when they signed up and, in some cases, buying licenses for the service.
How does the self-service signup work?
When someone signs up using a work or school-related email address, such as firstname.lastname@example.org, the domain portion of that email address (contoso.com, in this example) is used as their Office 365 domain name.
All other users who sign up with an email address on that same domain (say, email@example.com) are added to the same account.
Both Sara and Rob can use the service they signed up for, but they can't manage the service. For example, they can't purchase more licenses, add new users, or enforce policies on other users.