Nikon D80 is not taking pictures, WHY?

My Nikon D80 is not taking pictures. The battery is fully charge. When I press the button to take pics there's fashing 'f' and r15 appears. When the camera is on but not used 4.0 and k above the 4.0 appears.

There are some fail-safe features on your camera.

It will not take a picture if the lens is not in focus or if there's no memory card in the camera

Look on page 5 of your user manual to see what those letters and numbers mean

The "K" means that there are over 1000 shots left on the card. 4.0 K means over 4000 shots. This means that you have your camera set to too low a resolution. For best results, the settings should be Large, Fine. Look on page 35

The other "K" refers to colour temperature in degrees Kelvin and is used when you know specifically what colour temperature the light source is.

The flashing "F" tells you that you need to remount your lens. See page 132

And finally the r15 tells you the buffer capacity

Why does my monitor in Nikon D90 flashes with 'fEE"