In order to manage a META business account, your personal Facebook profile must have administrator rights there. Only then, you can add numbers on the WhatsApp Business API to the META business account.
In the following, you will learn step-by-step how to check whether your Facebook profile has the necessary administrator rights:
Log in to the Meta Business Manager
Select your META business account
Now look in the People tab on the left for the permissions of your Facebook profile
Not an admin?! Here you can find out how to become one.