When I try launching the game, I get this error: "attempt to index field '?'"
My only option is to disable it. What causes this problem?

Many things could possibly cause this issue. It's most likely another mod doing something radical, causing mine to trip over.

I'd need more information to say anything further, even just giving me the complete error message would help me narrow down exactly what is tripping over what.

"attempt to index field '?'" basically just says "I expected a data table, but it's not there.", the extra information in the error message usually gives details, such as mod line number, or possibly even the name of the parent table, which is vital information for me to give you advice, or fix a problem.

In summery, your message basically says "I found an error, but I'm not going to tell you what it is. Please fix it.".

A screenshot of the error message would help. The log file would be even more helpful.

Sorry, I am new to Factorio Mods. I will shortly get a more comprehensive error message for you. If I could copy-paste the error, I would have done that. I'll have to type it manually it seems.

OK, the game seemed to load with this enabled today. But when I enabled Industrial Revolution 2, I got the error. I uploaded here: https://imgur.com/2IGJZBC
Is your mod incompatible with Industrial Revolution 2?

The issue seems to be that my mod assumes "basic-oil-processing" exists, which is a vanilla recipe. IR2 must be deleting it.

This code is part of the oil overhaul (global) changes list. You can try to disable oil overhauls in my mod, then turn IR2 back on to see if it works.

Though, you should be aware that IR2 is one of those mods that likes to just ignore compatibility with other mods, so even if I fix this issue, other things might break.

Ok, thank you. I think I might just have to live without IR2.