I’ve been modeling this off and on most of the day and when I came to add in a UV sphere I wanted to chop up, all the settings for it were greyed out, it’s doing it with all of the Add Mesh > Primitives options and includes the F6 menu, is it something simple (like me being thick,) or a bug? I’m scratching my head a bit here. Bottom left you can see the greyed out menu 
Weird, after switching on the computer this morning and opening up blender in exactly the same state, the options aren’t greyed out any more, I’m assuming this is a bug now, I’m unsure how I should go about reporting this?
i had the exact same problem a while back and a restart fixed it. i wouldnt worry about it. and besides, if you have no idea what causes it, what can the blender developers do about it?
edit: point being, dont bother reporting unless you can recreate the problem for the devs