Update to 26.2 Please

java.lang.RuntimeException: Mixin transformation of net.minecraft.class_2275 failed
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:427)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:323)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1027)
at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.defineClassFwd(KnotClassLoader.java:160)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:355)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
at net.minecraft.class_2358.method_10199(class_2358.java:336)
at net.minecraft.class_2966.method_12851(class_2966.java:53)
at net.minecraft.client.main.Main.main(Main.java:170)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:470)
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: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:392)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:234)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:422)
... 18 more
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [allstackable.mixins.json:MixinCauldronBlock from mod allstackable] from phase [DEFAULT] in config [allstackable.mixins.json] FAILED during APPLY
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinError(MixinProcessor.java:638)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinApplyError(MixinProcessor.java:589)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:379)
... 21 more
Caused by: org.spongepowered.asm.mixin.injection.throwables.InvalidInjectionException: Critical injection failure: @Inject annotation on cleanStackedShulkerBox could not find any targets matching 'Lnet/minecraft/class_2275;method_9534(Lnet/minecraft/class_2680;Lnet/minecraft/class_1937;Lnet/minecraft/class_2338;Lnet/minecraft/class_1657;Lnet/minecraft/class_1268;Lnet/minecraft/class_3965;)Lnet/minecraft/class_1269;' in net.minecraft.class_2275. Using refmap AllStackable-refmap.json [PREINJECT Applicator Phase -> allstackable.mixins.json:MixinCauldronBlock from mod allstackable -> Prepare Injections -> -> handler$zzi000$allstackable$cleanStackedShulkerBox(Lnet/minecraft/class_2680;Lnet/minecraft/class_1937;Lnet/minecraft/class_2338;Lnet/minecraft/class_1657;Lnet/minecraft/class_1268;Lnet/minecraft/class_3965;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfoReturnable;)V -> Parse]
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.validateTargets(InjectionInfo.java:658)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.findTargets(InjectionInfo.java:589)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.readAnnotation(InjectionInfo.java:330)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.(InjectionInfo.java:316)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.(InjectionInfo.java:308)
at org.spongepowered.asm.mixin.injection.struct.CallbackInjectionInfo.(CallbackInjectionInfo.java:46)
at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:486)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo$InjectorEntry.create(InjectionInfo.java:149)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.parse(InjectionInfo.java:710)
at org.spongepowered.asm.mixin.transformer.MixinTargetContext.prepareInjections(MixinTargetContext.java:1329)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.prepareInjections(MixinApplicatorStandard.java:1053)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.applyMixin(MixinApplicatorStandard.java:395)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:327)
at org.spongepowered.asm.mixin.transformer.TargetClassContext.apply(TargetClassContext.java:422)
at org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:403)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:363)
... 21 more

Start by reproducing the launch failure and inspect allstackable.mixins.json, especially MixinCauldronBlock and its cleanStackedShulkerBox injection. Compare the target method reported in the stack trace with the current Minecraft 26.2 mappings; done means the mod launches without this MixinApplyError.

Written by the indexing model from the issue text.