It just crashes.

---- Minecraft Crash Report ----

WARNING: coremods are present:
offlineskins (OfflineSkins-1.12.2-v6-core.jar)
Contact their authors BEFORE contacting forge

// This is a token for 1 free hug. Redeem at your nearest Mojangsta: [HUG]

Time: 7/13/21 1:12 PM
Description: There was a severe problem during mod loading that has caused the game to fail

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Minecraft Chromium Embedded Framework (mcef)
Caused by: java.lang.UnsatisfiedLinkError: Can't load library: /Users/Boaz/Library/Application Support/minecraft/1.12/jcef/jcef_app.app/Contents/Java/libjcef.dylib
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1824)
at java.lang.Runtime.load0(Runtime.java:809)
at java.lang.System.load(System.java:1086)
at org.cef.CefApp.startup(CefApp.java:487)
at net.montoyo.mcef.client.ClientProxy.onInit(ClientProxy.java:143)
at net.montoyo.mcef.MCEF.onInit(MCEF.java:62)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:637)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
at com.google.common.eventbus.EventBus.post(EventBus.java:217)
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:219)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:197)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
at com.google.common.eventbus.EventBus.post(EventBus.java:217)
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:136)
at net.minecraftforge.fml.common.Loader.initializeMods(Loader.java:749)
at net.minecraftforge.fml.client.FMLClientHandler.finishMinecraftLoading(FMLClientHandler.java:336)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:535)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:378)
at net.minecraft.client.main.Main.main(SourceFile:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- System Details --
Details:
Minecraft Version: 1.12.2
Operating System: Mac OS X (x86_64) version 10.13.6
Java Version: 1.8.0_74, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 207942248 bytes (198 MB) / 536870912 bytes (512 MB) up to 2147483648 bytes (2048 MB)
JVM Flags: 7 total; -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M -Xmx2048m
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.42 Powered by Forge 14.23.5.2855 Optifine OptiFine_1.12.2_HD_U_G5 7 mods loaded, 7 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored

| State | ID           | Version      | Source                        | Signature                                |
|:----- |:------------ |:------------ |:----------------------------- |:---------------------------------------- |
| LCHI  | minecraft    | 1.12.2       | minecraft.jar                 | None                                     |
| LCHI  | mcp          | 9.42         | minecraft.jar                 | None                                     |
| LCHI  | FML          | 8.0.99.99    | forge-1.12.2-14.23.5.2855.jar | e3c3d50c7c986df74c645c0ac54639741c90a557 |
| LCHI  | forge        | 14.23.5.2855 | forge-1.12.2-14.23.5.2855.jar | e3c3d50c7c986df74c645c0ac54639741c90a557 |
| LCHE  | mcef         | 1.31         | mcef-1.12.2-1.31.jar          | None                                     |
| LCH   | offlineskins | 1.12.2-v6    | OfflineSkins-1.12.2-v6.jar    | aaaf83332a11df02406e9f266b1b65c1306f0f76 |
| LCH   | webdisplays  | 1.1          | webdisplays-1.12.2-1.1.jar    | None                                     |

Loaded coremods (and transformers): 

offlineskins (OfflineSkins-1.12.2-v6-core.jar)
lain.mods.skins.init.forge.asm.ASMTransformer
GL info: ' Vendor: 'ATI Technologies Inc.' Version: '2.1 ATI-1.68.25' Renderer: 'AMD Radeon HD 6750M OpenGL Engine'
OptiFine Version: OptiFine_1.12.2_HD_U_G5
OptiFine Build: 20210124-142939
Render Distance Chunks: 12
Mipmaps: 4
Anisotropic Filtering: 1
Antialiasing: 0
Multitexture: false
Shaders: null
OpenGlVersion: 2.1 ATI-1.68.25
OpenGlRenderer: AMD Radeon HD 6750M OpenGL Engine
OpenGlVendor: ATI Technologies Inc.
CpuCount: 4

it might also be that my computer is 10 years old
(ignore the offline skins mod, my friends play with a cracked launcher)

I can't test it easily but I suspect it's probably your hardware/outdated OS like you said.

I can test macOS 10.13 eventually. For now, 10.15 or 11 are recommended as that's what I built it for

Oh. I just used the original minecraft launcher instead of multimc. That worked.