reverse-factory/func.lua:425: attempt to concatenate a nil value
stack traceback:
reverse-factory/func.lua:425: in function 'makeFluidRecipe'
reverse-factory/func.lua:387: in function 'makeFluidItem'
reverse-factory/data-final-fixes.lua:110: in function 'final_fixes'
reverse-factory/data-final-fixes.lua:170: in main chunk
will this information help?

Hm, do you have any mods that add new fluids to the game? The error specifically references a line where I try to keep the solid-fluid conversion recipes in a tidy little group of the crafting menu/Factoripedia, by appending the order of the base fluid to the order of the ice-melting recipe (thus preserving the original order whenever possible)

Either the new mod-added fluid has no order and I need to rewrite the line with better handling for that case, or the ice-melting recipe somehow no longer has an order, and I would need to rewrite that line to catch that particular case.

In order to resolve the issue, I would need to know which mod causes it, because playing with just Reverse Factory and Space Age, I don't get the error.

I did use some mod like Tiberian Dawn and 2 Yuoki mod，that may cause what you mentioned about some fluid adding stuff.

I have the same error happening to me and I used the --dump-data to see all fluids from my game and indeed some of them have no order defined. I can still track which mod is from but that won't stop others authors adding fluids without order.

I installed a bunch of mods now that's why I didn't know where it came from but was not the mods mentioned above, disabling reverse factory made the game load normally

It would help to have the name of a mod to test with, since I otherwise can't tell for sure if the changes I make have solved the problem. Also sorry for delaying on this for so long, been having some irl issues to deal with, but they should be resolved by now.

I released an update (v9.0.22) that fixed a very similar error posted by another user here

If either of you were using Space Age Galore, then it's likely that the error was resolved by this update, but if it has not, then please respond with the name of the mod(s) that you believe might be causing the error, in order for me to verify and fix.

No problem, I'm just testing mods out to make a mod list for me for my next playthrough with SA with mods now that I think most of them is working well and are balanced (except some big overhaul ones), I'm currently playing DSP so I'm not in a hurry.

Yeah, that resolved the error at startup and I indeed installed Space Age Galore to test it out when the error appear for me.
Thank you for fixing it