When you decided to start verification, you may find that the button looks like this:
If this is the case for you, here are a few options for enabling it:
a) You can try creating an ad campaign with a link to WhatsApp Business. After you create the campaign the button should be enabled.
b) If it still isn't enabled, go to developers.facebook.com/apps and create an app. You do not need to create a working app. This process is simply to enable the button:
Before saving, copy the ID for your app. You'll need it later.
Finally, after saving all the information for your app, go to https://business.facebook.com/settings/apps/ and add the app you just finished created. Here's where you'll need the ID for your app.
After added the application to your business, wait 10 minutes and the button should be enabled. Don't forget to refresh your browser if the button still isn't enabled after a few minutes.
c) If even after trying these steps the button still isn't enabled, please contact your Onboarding executive to discuss more options.