Somehow, I have blocked the ability to add blocks to the ForThisSite and ForThisPage folders. "New Block" is grayed out. If I go to AccessRights, at least for the ForthisSiteFolder I should have Read, Create, Change, Delete, Publish and Administer on that folder. Not sure what has changed?
If you have access to these folders then make sure "Language Editing" is enabled for the particular language that you are editing.