Hello,
You might see the error “We couldn’t find an account with that username” if your account was deleted due to inactivity, meaning you had not signed in for more than two years. (OR)
This error may also appear if you manually closed the Microsoft account or removed the @gmail.com alias from it.
However, if none of these situations applies to you and you did not make these changes, your account may have been compromised.
- See if there are any unusual activity in your Microsoft Account.
- Check your recovery email address for any notifications from the Microsoft Security team about your account.
If your account has been compromised, you will have to submit an 'Account Recovery' request through the Xbox support team here. Make sure you enter all the details correctly, then click Submit. This team might help you recover your account even if the account information has been changed.
https://support.xbox.com/en-US/forms/recover-your-microsoft-account