Managing personal Outlook.com account settings, security, and privacy
Hi Leonardo,
Thank you for posting your problem i am here to help you today. Possible the reason remove account button is missing you are using a Work or School profile.
Try this step to remove your account from Microsoft Authenticator.
Using IOS
Go to Settings
General
Storage
Authenticator
Offload App
Using ANDROID
Go to Settings
Account
Manage Account
Tap and remove your account registered to your Microsoft Authenticator.
For more details you can check this article from Microsoft: remove all accounts from Microsoft Authenticator
Let me know how this goes. Looking forward to helping you out. If you need more help please feel free to let me know.
Best regards,
Jansher