This is usually because you’re trying to display posts from a personal Facebook profile. The plugin can only be used to display posts from a public Facebook page or open Facebook group. The easiest way to check whether you’re trying to display posts from a personal profile instead of a page or group is to view your page and see whether you have ‘Friends’, ‘Likes’ or ‘Members’. Profiles have ‘Friends’, pages have ‘Likes’ and groups have ‘Members’. If you have ‘Friends’ then it’s not a page or a group, it’s a personal profile.
If you’re using the profile to represent a business, organization, product, public figure or something else other than yourself, then we’d advise converting your profile to a page per Facebook’s recommendation: http://www.facebook.com/help/175644189234902/, as there are many advantages to using pages over profiles. It is in fact against Facebook policies to use a profile to represent something other than yourself (see ‘Why should I convert my profile to a page’ in the link above).
Once you’ve converted to a page then the plugin will be able to retrieve and display all of your posts.