You must be already having a Business manager account for managing your Facebook pages. But most probably it might not be a verified business manager.
The reason why you need to verify your business manager is to tell Facebook that it officially represents your business which in turn helps Facebook to give access to some restricted areas such as approving API requests for Whatsapp etc..
1) In order to start the process of getting your business manager account approved or verified, first go to your Business Manager account:
2) Click on the business manager account you are looking to verify
3) In the dashboard, scroll down and click on "Business Info"
4) Now, if your business is not verified by Facebook it will show an "Unverified badge". To proceed, click on "View Details"
5) Now comes the tricky part; most probably the "Start verification" button under Business verification section would be greyed out or would not work or might not be clickable.
6) In order to make that "Start verification" button clickable or working, you have to connect a Facebook app ID to your business manager account. In order to do so, first, go to "Facebook for developers"
7) Then on the "My Apps" dropdown, if you already have an app click on it or if you don't have an app to create a new one.
8) Then go ahead and copy the "APP ID" from the dashboard.
9) Now go back to Business manager click on "APPS"
10) Select "Connect an app ID"
11) Now paste the "APP ID" in the box
13) Then go back to step 5 page and now the button will have turned clickable and not greyed out.
14) It will now redirect you to a form in which you have to upload your basic business details.
After applying for verification, it might take 3-5 business days to get a reply back from Facebook. If everything goes with the Facebook policies, your business manager will get approved. This will help in unloading more awesome features such as Whatsapp business integration. You can buy me a coffee if you want me to do all this verification process for you :)