You need to have your phone number verified in order to use SMS 2fa.
To enable Two-Factor Authentication (2FA) via SMS, you need to click on "Settings" then "Enable 2FA via SMS" button. You will then be prompted to enter the one-time six-digit passcode that has been sent to your computer.
Upon receiving the code, enter it on the screen and click on "Submit". If you have not received the code on your mobile phone, please press the Resend OTP.
Your account will now have 2fa enabled. Each time you log in you will be required to enter a one-time passcode that is sent to your phone.