The new WordPress SEO plugin from Yoast version 3 may leave an error on your WordPress dashboard saying “Your homepage cannot be indexed by search engines. This is very bad for SEO and should be fixed.”
This is an error in itself and your page should be fine, to remove the error find on the WP Dashboard itself (not the Yoast Settings) a Yoast SEO Overview panel, here just click on fetch the current status and the error will be resolved. If the error still persists then the issue is something else.
This all seems to be the result of a collaboration between Yoast and OnPage.org – you now can opt out of this service which will stop any notifications for it, the checkbox to tick is in Yoast SEO Settings – General > On.Page.org