Tmux horizontal split isn't working, instead it brings up a weird menu

So I use ctrl + a (bound to a instead of b), and I try to ctrl + a and release then type in SHIFT + ' which is " so ctrl + a + " like the horizontal split should in theory work. When I do that I get a weird menu (I'm not sure what this menu is, why it pops up, I'm trying to figure out how to correctly get the horizontal split to work)