This adapter has no electrical communication with the lens and the camera thinks there is no lens attached. The camera has a setting to enable or disable shooting without a lens attached. You should really get the proper adapter with electronic contacts so that you can control Auto Focus and Aperture with the EF 75-300 lens.

In the mean time you can enable shooting in the Custom Function menu.

First, make sure the camera in not in the green "A" mode. Then look in the camera menu for Custom Functions and under CF II there will be an option for shutter release when no lens is attached. 

LINK: release the shutter when a lens is not attached