Just bought a new desktop, it has a Radeon RX550 graphics card, I have Poser 11.3.818 Pro, in render settings "Superfly" it's just showing Options - Intel Core i7 CPU and is not giving me the option to choose the graphics card, I thought that Poser Pro 11 had this option, appreciate any advice.

Try updating the drivers.  Otherwise, if no one in the forum knows, you may need to address this with the tech support.

A quick install of the free Blender 2.91 or higher would at least tell you if it's just due to Poser 11, or if it is some system-wide problem in seeing the new card. If it's a new desktop, that probably also means a default install of Windows 10, which I'm told often has its own problems - including too-tight initial security and driver permissions.

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Learn the Secrets of Poser 11 and Line-art Filters.

A quick look at the speck page for P11 states that CUDA enable device required for hardware accelerated rendering. Cuda cores aren't exactly the same as AMD Stream Processors.

https://www.posersoftware.com/store/products/17

I have an Nvidia card in my machine and win 10 automatically assigns that card to Poser usage. There should be a control panel app for your AMD card that allows you to assign CPU or GPU to a dropdown list of applications... in the Nvidia Control Center it's under 3d settings. That is if 11 can utilize AMD cards?

Again; AMD not supported for hardware accelerated rendering in P11 according to info supplied above from product page. CUDA is required.  https://www.posersoftware.com/store/products/17

Thanks for all your suggestions, I've done the driver update, tested Blender which works with the graphics card OK, downloaded some AMD software that controls the card, I'll check out CUDA which I know nothing about, will do some searches, I'll also check out security and driver permissions.

CUDA cores are Nvidia technology.

AMD uses Stream Processor technology.

They are not the same thing.

Hardware rendering in Poser requires CUDA. That means AMD cards are NOT supported for hardware accelerated rendering.  I've linked the product description twice now. All the drivers, permissions, searches and what not you care to try will not assist in getting an AMD card to do hardware accelerated rendering in Poser.

FWIW, even with CUDA, that generation of graphics card is easily over-whelmed by a middling-complex scene, "Huh ? Why is my render blank ??"

Aaaand you're back to CPU rendering.

Which is sorta how I ended up with a network render 'Box', as significantly cheaper and much more future-resistant than upgrading either of my twin GPU cards...

{ Shakes head...}

Thanks for all your suggestions, I've done the driver update, tested Blender which works with the graphics card OK, downloaded some AMD software that controls the card, I'll check out CUDA which I know nothing about, will do some searches, I'll also check out security and driver permissions.

Dear OP, why don't you listen to parkdalegardener here above?

It's time to replace your AMD Radeon with any nVIDIA CUDA GPU... 

All is not totally lost, if you want to keep the AMD card. You could still render from Poser using the Reality plugin, which with the free AVfix still runs with Poser 11. Scenes from Poser can also be sent to Vue, and Vue appears to be graphics-card agnostic.

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Learn the Secrets of Poser 11 and Line-art Filters.

So you know the why about this. The render engine SuperFly is based on is Cycles, same as Blender. Cycles was specified to use AMD's OpenCL technology to implement hardware rendering for AMD GPUs. AMD has dropped development on OpenCL. Until AMD provides an API for 3rd party hardware rendering it will never happen. At one point Cycles had OpenCL implemented and sort of working. But, the OpenCL API was so bad that it was no faster than a mid range CPU, probably why AMD gave up on it.

As NikKelly mentioned you need a truly modern GPU for it to be of any use. No less than 8GB of dedicated memory and it really should be an RTX card to give you access to the next level of speed Optix rendering. Optix is again an Nvidia thing but it's like 40x faster than a high end CPU. Forty times faster. Hours become minutes. Of course finding an RTX in stock is near to impossible and then it will be priced 3 times the list price.

This is why I have been yammering on about the AMD Pro Render Engine for a couple of years now.  It has a Cycles plug-in and in GPU agnostic.

Poser - Cycles Plugin - AMD Pro Render Engine.

Uses AMD Cards, Uses Nvidia Cards, will also use CPU and System memory, all at the same time.