Ultra 18k space skies, are 162,000,000 pixels in size, which is a VERY big amount of data to be stored in VRAM (the RAM for your GPU).

We recommend between 12gb and 16gb of RAM or VRAM to be able to render using these 18k worlds. Unfortunately, it's not possible for us to compress these images any further without losing quality, and having artifacts such as 'Banding' appear. 

The Ultra skies are really for people who are creating super high resolution still images, of 4K or above, where you will be able to see the fine details of the sky, in those images. 

This, due to how Blender manages it's memory pool, does mean that you would be expected to have a high end GPU with a lot of VRAM, or render on you CPU, where Blender managed memory a little better. We still recommend having 16gb of system RAM however, as a minimum specification, as this is the same RAM that will be running blender, your OS and other background processes.