WebGL build fail: assemblyResolutionException: Failed to resolve assembly: 'UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'

I'm trying to build a WebGL version (Windows and Android worked fine) and I keep getting this error. I'm in version 2020.2.1f1 and using Mirror as Networking.

assemblyResolutionException: Failed to resolve assembly: 'UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' Mono.Cecil.BaseAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference name, Mono.Cecil.ReaderParameters parameters) (at :0) Mono.Cecil.BaseAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference name) (at :0) Mono.Cecil.DefaultAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference name) (at :0) Mono.Cecil.MetadataResolver.Resolve (Mono.Cecil.TypeReference type) (at :0) Mono.Cecil.ModuleDefinition.Resolve (Mono.Cecil.TypeReference type) (at :0) Mono.Cecil.TypeReference.Resolve () (at :0) UnityEditor.MonoAssemblyStripping.DoesTypeEnheritFrom (Mono.Cecil.TypeReference type, System.String typeName) (at :0) UnityEditor.MonoAssemblyStripping.CollectBlackListTypes (System.Collections.Generic.HashSet`1[T] typesToPreserve, System.Collections.Generic.IList`1[T] types, System.Collections.Generic.List`1[T] baseTypes) (at :0) UnityEditor.MonoAssemblyStripping.GenerateLinkXmlToPreserveDerivedTypes (System.String librariesFolder, UnityEditor.RuntimeClassRegistry usedClasses) (at :0) UnityEditorInternal.AssemblyStripper.RunAssemblyStripper (UnityEditorInternal.UnityLinkerRunInformation runInformation) (at :0) UnityEditorInternal.AssemblyStripper.StripAssemblies (System.String managedAssemblyFolderPath, UnityEditorInternal.BaseUnityLinkerPlatformProvider unityLinkerPlatformProvider, UnityEditorInternal.IIl2CppPlatformProvider il2cppPlatformProvider, UnityEditor.RuntimeClassRegistry rcr, UnityEditor.ManagedStrippingLevel managedStrippingLevel) (at :0) UnityEditorInternal.IL2CPPBuilder.Run () (at :0) UnityEditorInternal.IL2CPPUtils.RunIl2Cpp (System.String stagingAreaData, UnityEditorInternal.IIl2CppPlatformProvider platformProvider, System.Action`1[T] modifyOutputBeforeCompile, UnityEditor.RuntimeClassRegistry runtimeClassRegistry) (at :0) UnityEditor.WebGL.WebGlBuildPostprocessor.CompileBuild (UnityEditor.Modules.BuildPostProcessArgs args) (at /Users/bokken/buildslave/unity/build/PlatformDependent/WebGL/Extensions/Unity.WebGL.extensions/BuildPostprocessor.cs:365) UnityEditor.WebGL.WebGlBuildPostprocessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args) (at /Users/bokken/buildslave/unity/build/PlatformDependent/WebGL/Extensions/Unity.WebGL.extensions/BuildPostprocessor.cs:912) UnityEditor.Modules.DefaultBuildPostprocessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args, UnityEditor.BuildProperties& outProperties) (at :0) UnityEditor.PostprocessBuildPlayer.Postprocess (UnityEditor.BuildTargetGroup targetGroup, UnityEditor.BuildTarget target, System.String installPath, System.String companyName, System.String productName, System.Int32 width, System.Int32 height, UnityEditor.BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.Build.Reporting.BuildReport report) (at :0) UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)