Duo Security is a method of strong authentication that allows a third party service to send a set of validation codes known as passcodes to your device. There are a number of reasons one of those codes may not work.
You might have already used the validation code.
The screen will tell you which code it is expecting next from the list of codes.
You might have requested a new set of validation codes.
This invalidates all the codes in all of your earlier sets.