I am not familiar with this exact lens and camera model, but with similar questions related to Sony Alpha cameras and manual focus lenses. I assume your camera gives an error message similar to the following: "Check the lens attachment. If the lens is not supported, you can permit use of the lens in the custom menu."

I suspect the reason for this is that the Rokinon 85mm lens does not have any electronic contacts (as it is a manual focus lens and these contacts are mainly used for auto focus) and the camera thus does not 'see' the lens (because the camera 'sees' the lens by communicating electronically with the lens via these contacts). See also http://community.sony.com/t5/Alpha-SLT-DSLR-Cameras/A77-Lens-Compatibility/td-p/27585 (On a side note, the contacts I mention are the five golden squares in this picture of an A-mount lens, whereas this picture of the Rokinon 85mm shows that it doesn't have any.)

In order to overcome this problem, you can enable the "Release w/o lens" option as follows:

MENU button, then "cog wheel" 1, then Release w/o Lens, then Enable

See Sony a77 manual, p. 175. This way, you're telling the camera it's ok to release the shutter even if it thinks no lens is attached.

By the way, as the Rokinon 85mm is a manual lens, it's very worthwhile to look at the manual focus support of the Sony a77, see the manual, p. 123. In particular focus peaking is an extremely useful feature.