The most common cause for "Incorrect Code" error is that the time on your Google Authenticator app is not synced correctly. To make sure that you have the correct time in your Google Authenticator app follow the instructions for your operating system below.
Go to the Main Menu on the Google Authenticator app
Select Time correction for codes
Select Sync now
The app will confirm that the time has been synced, and you should now be able to use your verification codes to sign in.
On iOS (Apple iPhone):
Go to the iPhone Settings App. (your phone settings area)
Select Date & Time
Enable Set Automatically
If it's already enabled, disable it, wait a few seconds and re enable
This should solve the error you are receiving. And if you still cannot resolve the error, please feel free to write to the STEX Support Team.