When you are entering your phone number, you need to ensure that you are using the international format without any spaces or other characters.
EG +15551234.
Once you are done, you can go to Settings and activate Two-factor verification (2FA) which provides further security to your account.
If you would like to set up Two-factor verification (2FA) Authenticator App: Tap here.
If you would like to set up Two-factor verification (2FA) via SMS: Tap here.