next_upgrade is a tag used for the upgrade planner tool. The tool itself can be configured to upgrade any entity into any other entity that has some things in common... collision box size and "fast replacable group" are the main 2.
As such, I set "data.raw.boiler.boiler.fast_replaceable_group = "boiler"", and also set it on the heat exchanger and all other boilers. I then add next_upgrade = "boiler-2" to the boiler, and so on, so that the upgrade planner knows to change the boiler into a boiler-2 by default.
Now, the problem with this is that you NEED the fast_replaceable_group to be the same across all boilers for it to work, but then there's this problem... Other mods.
Some other mod is probably setting the fast replace group to something else on the vanilla boiler, which causes the game to not know how to handle the next_upgrade tag, and you get this error.
The interesting thing is that you only have bobpower listed in the "Mods to be disabled" section, if all you had modifying the boiler was bobpower, there shouldn't be an issue.
If you just want to make everything work, unzip the mod into your mods folder, and then find the file prototypes/entity/boilers.lua, open it in some kind of text editor (Wordpad works if you're using windows) and scroll right down to the end of the file where you'll find the line:
data.raw.boiler.boiler.next_upgrade = "boiler-2"
Delete that line and save.
The Upgrade planner won't know what to do with the boiler anymore, but the game should load.
You'll need to delete the zip file too.
Don't know where your mods folder is? This might help: https://wiki.factorio.com/index.php?title=Application_directory#Changing_the_application_directory
It'll be in different locations depending what OS you're using, and how you play the game. Since I download it off the factorio website, it's just inside my factorio program folder, but you're probably playing from steam, and that's in %Appdata% on windows.
