I cannot find the "Massive Assembler". I created a new game and opened the editor, and only found the Massive Chemical Plant and Massive Electric Furnace—there was no sign of the Massive Assembler. I've scanned a large area around the map; I even found a legendary-quality electric furnace, but not a single Massive Assembler.
Additionally, I suggest that the mod should recommend players choose "Railworld" when creating a new game, so that the positive benefits brought by the mod can be balanced by using more distant ore veins. Alternatively, players could choose "Island" when creating a game, and limit the number of "massive production buildings" generated based on island size to balance the gameplay.
This mod reminds me of Tesla's Gigafactory in Shanghai, which was the world's first Gigafactory. Thank you for sharing this.

The spawning of entities is random. Maybe you just got unlucky. I once had a map with only one coal deposit; no more spawned. If the map contains more than just furnaces, the mod works correctly.

I actually discovered the problem—I was playing with the machines spawn probabilities and lost. I'll fix it in the next update (today).

I've tested and found that this mod conflicts with "Krastorio 2", which causes the error.
https://ibb.co/4w33FP13

Krastorio 2 has issues with smelters—the mod will replace them with assembly machines. I'll figure out how to integrate my mod with Krastorio 2, but for now, consider these mods incompatible.

Thanks for your reply