java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'difficultyplus'!
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.lambda$invoke0$0(EntrypointUtils.java:51)
at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:49)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke(EntrypointUtils.java:35)
at net.fabricmc.loader.impl.game.minecraft.Hooks.startClient(Hooks.java:52)
at net.minecraft.class_310.(class_310.java:443)
at net.minecraft.client.main.Main.main(Main.java:198)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:461)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
Caused by: java.lang.NoClassDefFoundError: website/skylorbeck/minecraft/skylorlib/MidnightConfig
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
at java.base/java.lang.ClassLoader.defineClass(Unknown Source)
at java.base/java.security.SecureClassLoader.defineClass(Unknown Source)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.defineClassFwd(KnotClassLoader.java:186)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:347)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:145)
at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
at website.skylorbeck.minecraft.difficultyplus.Difficultyplus.onInitialize(Difficultyplus.java:15)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:47)
... 7 more
Caused by: java.lang.ClassNotFoundException: website.skylorbeck.minecraft.skylorlib.MidnightConfig
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(Unknown Source)
at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:226)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:145)
at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
... 17 more

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

-- Head --
Thread: Render thread
Stacktrace:
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.lambda$invoke0$0(EntrypointUtils.java:51)
at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke0(EntrypointUtils.java:49)
at net.fabricmc.loader.impl.entrypoint.EntrypointUtils.invoke(EntrypointUtils.java:35)
at net.fabricmc.loader.impl.game.minecraft.Hooks.startClient(Hooks.java:52)
at net.minecraft.class_310.(class_310.java:443)

Tested on "Better Minecraft Fabric v19.1" modpack.

What version of Skylib are you using? Make sure its the most recent version

Was using an old version. Yes, this solved the problem. Thanks, @skylorbeck !