Solved it.
In the VulkanPC/DataDrivenPlatformInfo.ini under [ShaderPlatform VULKAN_SM6] I changed to BindlessSupport=AllShaderTypes instead of RayTracingOnly.
If it does not work on your side - start unreal with the command line and read the log messages.
This extension must be active VK_KHR_ray_tracing_position_fetch :
As described here, the corresponding nvidia-driver is necessary.
I could get it running with the nvidia-550 with sudo apt install.
Disable under targeted RHI the vulkan sm5, only sm6 should be active.