You receive the following error when trying to sign up for a Microsoft application that supports both personal and work/school accounts:

It looks like there's already an account with some.user@ttu.edu. If it's you, go back and sign in. Otherwise, choose another username.

Around 2016, Microsoft merged their cloud identity systems. Students and employees will not be able to use their work/school email accounts to create a personally owned Microsoft account (ending in @ttu.edu). However, some apps support the use of work/email accounts and would not require you to sign up for them again.

Sign in to the Microsoft application using your existing account instead of creating a new account.

Outcome

The message will no longer occur.