You most likely wont be able to do online check in due to it being an international flight (I've never had success with this myself), even though the first leg is domestic, so that is what it is.

As for picking a seat, but did you check to make sure there were seats available for choosing (and not those that are greyed out for Medallions or those that are already reserved?)? That maybe the problem. My hunch is that this is just another one of Delta's many IT problems, assuming there are still seats left to assign.