You must use a number that does not hava an account on any WhatsApp platforms (WhatsApp or WhatsApp Business API). If you want to use the number that is already associated with the WhatsApp Platform then the number has to be deleted from Facebook Business Manager.
For this, please follow the following steps:
Step 1: Go to your Facebook Business and Choose the Business Settings Page.
Step 2: Click on WhatsApp Accounts under the Accounts section.
Step 3: Choose the WhatsApp account.
Step 4: Go to Whatsapp Manager.
Step 5: You will land on this page.
Step 6: Click on delete.
Step 7: A confirmation box will appear where you can select Next.
Note: A number can not be deleted if the business has sent paid messages within the last 30 days using that number. If the business has sent paid messages in the past 30 days, you will be redirected to the Insights page showing the date of the last paid message.
For further details: https://developers.facebook.com/docs/whatsapp/on-premises/phone-numbers/
