Each code is valid for 10 minutes, works once and allows 5 attempts; requesting a new code cancels the previous one.
If the email doesn't arrive, check your spam and promotions folders, then select Resend code, which is available after 60 seconds. When the code is accepted, you are signed in automatically.
If you closed the verification screen, sign in with your email and password and choose Enter verification code. If the code still doesn't arrive, contact support. Never share the code with anyone, including FALKR support.