Thank you all for your very informative replies to my original post.

Since I've posted it I contacted the Poser support as well and they replied that I need a Nvidia GPU for Optix Rendering - what is kind of obvious.

Then I sent them the following quote from their own homepage and ..... they did not reply at all.

"Cycles supports GPU rendering with three GPU rendering modes: CUDA, which is the preferred method for older Nvidia graphics cards; OptiX, which utilizes the hardware ray-tracing capabilities of Nvidia's Turing architecture & Ampere architecture; and OpenCL, which supports rendering on AMD graphics cards."

From their homepage

Unfortunately there are very misleading informations on the official poser site! There is not a single hint, that the statement regarding AMD is not valid for the Mac version.