help me, how to make a cube grid dimension?

You can do i by first making the dimension all air by setting the default blocks to air(you'll get a error saying that you need a non air block but you can just open its element and select the "dimension" not the "dimension_type").

just like in this image

https://drive.google.com/file/d/11-P8c8Qg4Mnrvzk_3zMCej_Uqxl8jJuZ/view?usp=sharing

and we'll use structures as the repeating dimensions layout.

go to a world and give yourself a structure block and 16x{what ever height you want}x16 structure of blocks and save it(if you don't know how to use structure blocks just watch a tutorial of it, and also just search up where you need to find where your exported structure file is).

https://drive.google.com/file/d/1pjwFVRUKFIQQWJ6b5WM07Cbi-xfaztsz/view?usp=sharing

go back to MCreactor and make a new feature, in that feature grab "custom structure" and place in on Configured feature and also put the custom dimensions biome on the "Restriction biomes" or you'll get spawn the structure on any dimension

do note this is the lazy why on doing this because you can make it 16x16x16 and connect it vertically with jigsaw blocks so you can make variants of the sections