I tried to create a custom Scorpion mob, but it would spawn, even if its weight was set to 10000 (type creature).
So I decided instead to have Scorpion Nest blocks spawn, replacing sand. But they still won't spawn!
Issue comments
When possible, attach your MCreator workspace (or even better a minimal example workspace required to reproduce the bug), game logs (full logs, do not crop them, in text format, not screenshots of logs) from the Console tab or terminal and steps to reproduce the bug in your workspace. If logs are long, use service such as Pastebin to host the log and provide the link to the log from the ticket. Please specify which mod elements to check in your workspace too, if applicable.
This is mod, I need shareable zip workspace
https://www.youtube.com/watch?v=UzNvogqbHDg&feature=emb_title
It seems that setting ores to generate on the surface won't work, after all ores are generated in underworld generator stage where the floor might not even be generated yet.
I suggest you make a structure with your custom block of size 1x1x1 and use structure spawn to make your structure spawn on the ground in desired biomes.
It might have to do with group size set to 1 too, try increasing this or use my other solution.
This is unfortunately not enough details or context for any sort of help.