The code in the Authenticator app used for ESS will only be valid for 30 seconds. Make sure the code is entered into ESS in time.
The Authenticator app requires the clock on your mobile device to accurately report your local time. If your device clock is set to manual, then the code may register as invalid. Reconfigure your system clock to automatic. After updating your clock, restart your device and make sure the new time is set correctly.
For example, iPhone users can go to Settings, General, Date and Time to ensure Set Automatically is turned on. The steps required will differ depending on your phone. Refer to your phone's manufacturer for more details.