Facebook has 3 different Terms and Services that need to be accepted in order to connect your Ad Account. In some cases, these can be a bit hidden and this guide will walk you through what is necessary to get them accepted for each Ad Account you plan to connect.
https://business.facebook.com/customaudiences/app/tos?act=
https://www.facebook.com/ads/manage/customaudiences/tos.php?act=
https://www.facebook.com/ads/leadgen/tos?act=
In each of these links above you will notice at the end "act=" navigate to your business manager in Facebook and copy the Ad Account # from Facebook and place it here and visit each of these links to ensure you have accepted the TOS.
In this example, I would take 1345404199232275 and place that at the end like this: https://business.facebook.com/customaudiences/app/tos?act=1345404199232275
Make sure that the correct Account is also selected prior to accepting:
Once you have completed this please continue with the Integration.