Verify that your password has not expired by signing into Office.com
If you are able to sign into office.com but still get continually prompted to enter credentials in the Outlook app follow the steps below:
Sign in to the OWA portal (click HERE if this is your first time signing into the OWA portal)
Once signed in select the gear icon at the top right-hand corner of the window and select Options:
Select Mobile Devices under General:
Delete all the devices you aren't currently using by highlighting the device and clicking the minus (-):
Click Yes when asked if you are sure you want to remove the device:
Once you have removed all the devices you are not currently using, close and re-open all Outlook windows.
If the issue persists try recreating the Outlook profile.