If you are trying to use Two Factor Authentication Code for Sign In, ensure that:
The mobile number added is Canadian/American
You have network coverage in your area
Your mobile number on your profile is correct, you can confirm that under Settings >> Manage Profile >> User Settings
Sometimes, there could be network delays and you may not receive the Authentication Code immediately. The Authentication Code is valid for 5 minutes.
You might be interested in the following questions
1. How does Two Factor Authentication for Sign In work?
2. I am unable to see the email option for Two Factor Authentication for Sign In, what should I do?
5. I received ‘Too many login attempts’ error message, what should I do?