I'm trying to generate a map on my headless server, but the error of:

Error while running on_init: rso-mod/control.lua:1121: attempt to concatenate local 'richnessMod' (a nil value)

keeps hitting and fails the whole thing.

Seems that the line on 1121 in control.lua:

debug(res_name .. " allotment mod " .. allotmentMod .. " size mod " .. sizeMod .. " richness mod " .. richnessMod )

is causing the issue. Commenting it out fixes problem without repercussion.

What you are seeing is a symptom of different problem.
Can you do the following:
1. Add richnessMod = richnessMod or "" before this line
2. Change debug to log
Then run the server and post the usual factorio current.log on forums in RSO section.

The main issue seems to be that one of resources has richness defined as constant that RSO did not recognize. It might be specific to headless server and your configuration of it.

I'll have to try that once we reset the map again. I had thought it was the resource settings i had, but i generated a map from my desk top and tried to simply load it, same error. I reset the resource settings, same error. Nothing fixed it.

But i'll do as mentioned when im able.

If it also breaks on desktop then no need to play with logs - just post me the mods set on forums and I'll check if I can reproduce it.

Please grab 3.4.3 and post game log after running it. It should contain logging to show me which resource caused the problem and with what value.