If you received the error like:  Authentication failure:  Invalid username or password"  after you've correctly entered in your NetID and password, you maybe locked out of Duo Security two-step login.  This can happen with any system that requires two-step login.

Example of authentication failure:

Please call 2-9800 or submit a service ticket so that a Duo Administrator can unlock your account. 

Ensure that your default device is available to you when you log in and if necessary update your Duo settings so that you have a default device available: 

Managing Your Duo Devices and Settings