You have very high settings, and you just do not have enough RAM.
How to generate it with high settings:
1. Disable the generation of grass and trees, and reduce the texture size to a minimum.
2. Generate terrains.
3. Optional: Save each terrain in a separate scene.
4. Regenerate textures with the required resolution.
5. Generate the grass.
6. Generate the trees.
I strongly recommend to save the scene after each step.
P.S. I do not recommend to use Provider - Google for the high settings. There is a very big chance that you will be blocked.
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