I am having difficulty screwing the fans onto the radiator of my DeepCool AIO. The holes appear to be completely stripped out, and the screws are extremely difficult to screw in. I'm not sure what I am doing wrong.

The fan screw holes on the radiator are not threaded on purpose. When using the included self-tapping fan radiator screws in conjunction with the unthreaded radiator holes, a better/tighter hold is made. It may take a little more torque to tighten down those screws, but it should be possible.