NOTE: As of October 31, 2018 Facebook is requiring any ad account that contains a customer file as a custom audience to be hosted within a business manager.
What is a business manager?
Facebook's business manager is a dashboard that allows you to manage all of your assets (pages, ad accounts, people) in one secure location.
Why do I need a business manager?
Facebook's business manager is not only ideal for have a organized view of all your business' Facebook activity, but is also required for any ad account that has a custom audience that is a customer file (ie. Follow Up Boss database)
How do I create a Business Manager?
Related Article: How to set up a Facebook business page.