I found that this is due to a lack of vram. What card are you running?
Viewing post in FS25 Judith Plains, Montana 4X comments
CTRL + SHIFT + ESC , go to the performance tab and see what it says in the upper right corner for GPU.
Basically card probably can't handle this map so you can try lowering your resolution settings or turning ON DLSS/FSR.
This is a 4x map with some custom textures so it can be more demanding than a base game 1x map.