Solutions for Microsoft 365 login failure

Related article: "This PC isn't joined to a domain" error

1. Disconnect and reconnect your Microsoft 365 account. Go through the computer Start menu - Settings - Accounts - Access work or school - highlight your Microsoft 365 account - Disconnect. Then reconnect by going through +Connect, and enter e-mail address and password to log in.

2. The issue may be caused by corrupted user credentials so you can go to the Credential Manager to remove them completely:

Go to Control Panel - User Accounts - Credential Manager, select Windows Credentials - Find the Office related entries and remove them all.

Open a new Word document, enter the account and password, and check if the problem persists.

If the problem still occurs, try repairing Office from Control Panel - Programs and Features.

Some of the information used in this article was taken from the following link:

https://social.technet.microsoft.com/Forums/ie/en-US/233e9eb3-b40e-4148-93c6-ac5c4c881f59/why-do-i-have-to-login-to-my-microsoft-account-every-time-i-open-an-office-application-eg-word?forum=officesetupdeploy