I am trying to run ERM with a fair number of mods, including the Py suite, Rampant, Extended Evolution, most of Schall's...
Any idea what's causing this? I was going to look into replacing Zombie Horde with one of your factions, but I can't get it to launch.

enemyracemanager/prototypes/extend-defense.lua:131: attempt to index field 'mech-armor' (a nil value)
stack traceback:
enemyracemanager/prototypes/extend-defense.lua:131: in main chunk
[C]: in function 'require'
enemyracemanager/data-updates.lua:13 in main chunk

Check whether Enable All Feature Flags mod is enabled and space-age mod has disabled. It's most likely cause by that.

If you don't have "Enable All Feature Flags", check whether space-age has disabled. If it is, then some mods are messing around with space-age feature_flag.

Another scenario I can think of is that, a mod disables mech-armor and replaces it with something else.

I do have Enable Feature Flags enabled with Space Age disabled (in the mod manager, not Steam). This is necessary for running Py with decay. Is Space Age required for ERM? The in-game manager shows it as optional.
Py probably also replaces mech armor with its own version. I do also have extra armor from Frost Biters and Schall.
Any workaround for your mods to play nice with these?

that "Enable Feature Flags" mod set space_travel to true when that's not the case (space age disabled). Hence anything that based on that space_travel flag have potential to crash.

I can ignore mech-armor if it doesn't exist. But I don't guarantee there is no other crashes. That flag is used in many places in this mod.

space-age is optional when other mods doesn't mess around with feature_flags.