The phrase would mean different things, it may mean that there is a hole, it may also mean an open area suited for a certain purpose and most of all it may mean that a possible romantic feeling may arise on a person. Thanks i hope i add some information about this...

"Have an open spot" = have an employment position available to be filled. The sentence above says basically that all the positions for people to work at lunch time have been filled and they don't need anybody to work at that time period.