I cannot remove an existing alias on my account

Hi!

Is your email a custom email, such as @yourcompany.com.br, or @outlook.com / @hotmail.com?

Is your email personal, corporate, or educational? 

@Sam said

one of the alias on my account cannot be removed, as there is not REMOVE button next to it

Did you ever have a old Skype account? If so, it is probably your old Skype ID.

e.g. for my account, Hornblower409 was my old Skype ID

And Microsoft, in it's infinite wisdom, can't figure out how to remove a Skype ID without closing the whole Microsoft Account.

https://support.microsoft.com/en-us/skype/how-do-i-close-my-account-for-skype

•  A Skype identity cannot be detached from the Microsoft account.
• Closing the Skype account means closing the associated Microsoft account.