When you create a WhatsApp account, you’ll see two different screens:
Registration: This screen appears when you're creating a new account or reregistering your existing account. To confirm that you own the phone number, you’ll be prompted to enter a 6-digit registration code sent to you via SMS or phone call. Verifying your phone number with a registration code is the only way you can activate your account, and you must be able to receive the code on your phone.
Two-step verification: This screen appears after you successfully register your phone number on WhatsApp. Two-step verification is an optional feature that adds more security to your WhatsApp account. When enabling this feature, you create and confirm a unique PIN that’s required to access your account. The two-step verification PIN is different from the 6-digit registration code you receive via SMS or phone call. You can learn more about two-step verification in this article.
If you didn't enable two-step verification, but are prompted to enter a PIN, the previous owner of the phone number might have enabled it. In this case, you'll need to wait 7 days before resetting the PIN to access your account.
If you forgot your PIN and you've successfully registered your phone number on WhatsApp, tap Forgot PIN? > Disable to access your account.
Note: WhatsApp can only be used with one phone number on one device at a time. After you successfully register your phone number on WhatsApp, anyone who might be using your account is automatically logged out and your account is protected.