How to solve facebook start verification button greyed out

1. Make sure you have completed your business information on the settings -> About section of the business manager page
2. Turn on 2-factor authorisation for your admin users
3. If the option still doesn’t appear – go to the facebook developer page and create an app – https://developers.facebook.com/apps/
4. Associate the new app with your Facebook Business Page
5. In your App dashboard – go to settings and basic and scroll down to business verificatio
6. Click on verify here – for some absurd reason it works! – you will need to upload tax documents etc to verify

Reference : https://developers.facebook.com/community/threads/1010572709333176/