In Photoshop, make sure you're on a layer that can be filled (text layers, for example, can't be filled using the Fill command until they're rasterized) and make sure only one layer has been selected.
The "Shift-F5 not working" issue has also had this happen to me, as well, in Photoshop CC. One time it works, the next time it doesn't. It could be a memory issue. I would suggest saving your project, then exit the program and once you're back out to the desktop, reopen Photoshop. Open your project file and try the Shift-F5 function again.