Hi @Gerald Ho ,
Welcome to our forum!
It seems like you've been having some trouble with Outlook 2016/2021 constantly asking for your password and you have tried many troubleshooting steps.
Could you clarify when this issue first started and if any changes were made before it began? Also, does the password prompt only appear when you're logging in or does it enter the "password required" state without any interaction? If it's the latter, we recommend trying the method outlined in this document. Besides, you could reset the passwords of affected users. Additionally, it's worth checking if any firewalls or security software may be blocking the authentication process.
Hope the above information is helpful to you!
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