That card has the bare minimum amount of VRAM for After Effects to use it for GPU acceleration of the ray-traced 3D renderer. If you have any other applications running that use VRAM, then After Effects will not be able to initialize the CUDA-based renderer.
This is covered here:
OK, thanks I understand. Just one more question. How much VRAM would you recommend? I know "the more the merrier"is always a RAM consideration, but what do you recommend as a nice safe amount to work with? In my travels I had found an answer that indicated 1GB was barely enough, but I can't seem to find a base minimum ...(3GB, 4GB, 5GB?) ... for using this feature. My card has 2GB and since I can't upgrade my iMac, I might upgrade my PC and jump back over to that platform depending on the cost vs usefullnes.
The answer varies depending on what else you're running, what monitors you have connected, et cetera. You mentioned four VRAM-hungry applications in your original post. The easiest thing to do is to learn to shut down applications not necessary for your current work when you're not using them. That is the best advice for performance in any context.