i get this error on my windows desktop (running windows 10) and on two of my mac users (running OSX 10.14.6), but not one of them.

it reads:

Allomancy (allomancy) encountered an error during the common_setup event phase
net.minecraft.world.biome.Biome.func_222280_a(Lnet/minecraft/world/gen/feture:Lnet/minecraft/world/gen/feature/Feature;Lnet/minnecraft/world/gen/feature/IFeatureConfig;Lnet/minecraft/world/gen/placement/Placement;Lnet/minecraft/world/gen/placement/IPlacementConfig;)Lnet/minecraft/world/gen/feature/ConfiguredFeature;

here are the log files for the mac and the windows versions.
latestMac.txt
latestWindows.log

i have also included a log of where i did not have the problem:
latestMacSuccess.log

In the successful log, you are using Forge 31.2.9, in both unsuccessful you are using 31.2.0 - have you tried updating?

just tested it with 31.2.20 and id didn't work.

also didn't work with 31.2.9. same error

I haven't been able to recreate this issue. I'll try poking around some more

I've been unable to recreate this. I'm about to post another version with some minor tweaks, please try that

just tried it on a totally new computer and it worked. here is a log.
latest.txt

as of right now I am not reopening the thread as I have yet to be able to test the new mod on any of the user I know don't work.

just tested with my windows computer and it works!! here is the log if you want it:
latest.log