I am following along on the lessons for the rocket fin. I enabled 3D cursor, duplicated my fin, and then tried to rotate it into place, but it opens towards the back. See here: https://gyazo.com/6f0026cf60d284b2e4cdbcda361e92af
Looks like you accidentally moved your 3D cursor out of the center! To set it back to the world origin, press Shift + S in the 3D Viewport, and select “Cursor to World Origin” from the pie menu that pops up.