When adding the Facebook Widget, why do I get the error "This page does not have permission to install the custom tab"?

If you get an error stating you do not have permission to install a custom tab, this is likely due to the recent policy implemented by Facebook which states as follows:

"To ensure tabs lead to high-quality, accurate content, the Page Tabs feature is only available to Pages with 2000 or more followers, or pages managed by whitelisted apps."