My Nexus 5 won't update
From what I can read on the image you posted it looks like your device has 4.4.4 not 5.1.1, and possibly a custom rom rather than the stock one (it looks like omni_hammerhead/hammerhead: 4.4.4 in the screenshot).

If you have a custom ROM you will not be able to apply an OTA update as they will only work for the stock ROM.

Your best bet might be to flash a factory image (see my instructions here - you'll need to modify the file names but the process should be the same). You should note that unless you've already unlocked your bootloader this will wipe your device, and even if you have if you don't follow the steps correctly it still might (so you'll have to set everything up from scratch again unless you have a backup).