WhatsApp API has limits on what phone numbers you can use. Mainly, you can not have the same number on both WhatsApp app and Chatfuel automation. This helps Meta avoid spam from personal accounts and account fraud.
In other words, you must create a new WhatsApp account to use automations in Chatfuel. You’ll then need to import your contacts. This is a WhatsApp requirement and is not controlled by Chatfuel.