This can happen for several reasons:
1. Some countries have a slow SMS delivery speed. Wait for a couple of minutes.
2. One of the SMS services is temporarily down in your country
3. You entered a wrong phone number or selected an incorrect country. Make sure that you have provided correct information.
If this didn’t help, press the “Resend” button and a new confirmation code will be sent to the phone number. Note that there is a 30 second restriction between requests.