If you see the error "The requested data could not be found or you are not allowed to access" when attempting to share content with a page or community, it might be that the admins of the page or community have configured that only admins can post to the timeline for that page or community.

You can confirm this by visiting the page or community and checking the timeline app. Is there a "What are you working on" text box at the top? If not, the admins have turned off regular subscribers or members posting on the timeline.