First, your phone number needs to be added to your profile. Go to the upper left hamburger menu -> “My profile” -> Edit (the pencil icon in the upper right corner) -> Add your phone number. Be sure to enter your phone number with the country code (e.g., +1 (###) ###-####).
Next, click on “Not verified” from your profile page -> “Send me a verification code” -> Enter the verification code you will receive via SMS (text message).