In order of how often it is the answer.
The code has not arrived
Check the number, including the country code. One wrong digit sends the code to a stranger.
Try Send by SMS. If one channel is not reaching you, the other often does.
Wait before resending. Hammering the button does not help and can slow you down.
The code is valid for ten minutes. An old one will not work — request a fresh one rather than retyping the last.
The code is rejected
You get five attempts before it is locked and you need a new code. If you are sure the digits are right, you are probably typing an expired code.
You changed your phone number
This is deliberately not self-service — if it were, anybody who could reach your handset could move your account. Contact us and we will move it.
The account is blocked
Usually an unpaid win. How that works and how to clear it. If you think it is wrong, say so.
Questions people ask
What if I lose the phone my account is on?
Tell us straight away.
Can I sign in with email instead?
The account is the phone number.
Why is there no password?
A password is one more thing to lose, phish or reuse. A code to a verified number is both simpler and harder to steal.
More: how to create an account.