Facebook requires acceptance of different terms of service depending on the type of custom audience you are trying to create.
While AdEspresso usually automatically does this for you, sometimes it is not possible.
If you're having troubles creating new custom audiences, first check all the following links to see if you've already accepted Facebook's Terms of Service. If not, please accept them and try again:
If you own a business manger account too, you also want to double-check this link: