I’ve added the Engine and InputCore modules to the PhysicsCore.Build.cs:
using UnrealBuildTool;
public class PhysicsCore: ModuleRules
{
public PhysicsCore(ReadOnlyTargetRules Target) : base(Target)
{
PublicIncludePaths.Add("Runtime/PhysicsCore/Public");
PrivateDependencyModuleNames.AddRange(
new string[] {
"Core",
"CoreUObject",
"Engine",
"InputCore"
}
);
I’ve added the #include "Materials/MaterialInterface.h" to the top of my PhysicalMaterial.h file, and I’ve added the same variable that you did:
UPROPERTY(EditAnywhere, BlueprintReadOnly, Category = PhysicalProperties)
TSoftObjectPtr<UMaterialInterface> TestInstance;
There are way fewer errors now 
but it still won’t compile. 
Severity Code Description Project File Line Suppression State
Warning C4005 'TEXT': macro redefinition ... C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um\winnt.h 603
Error (active) E0145 member "FixedTagPrivate::FMapHandle::StoreIndexBits" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\CoreUObject\Public\AssetRegistry\AssetDataTagMap.h 110
Error (active) E0028 expression must have a constant value UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\CoreUObject\Public\AssetRegistry\AssetDataTagMap.h 114
Error (active) E0145 member "FBulkDataStreamingToken::InvalidOffset" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\CoreUObject\Public\Serialization\BulkData.h 132
Error (active) E0145 member "FBulkDataBase::InvalidToken" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\CoreUObject\Public\Serialization\BulkData2.h 125
Error (active) E1574 static assertion failed with "TAtomic is only usable with trivial types" UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\Core\Public\Templates\Atomic.h 543
Error (active) E0145 member "FName::StringBufferSize" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\Core\Public\UObject\NameTypes.h 471
Error (active) E0145 member "FNavigationLinkBase::InvalidUserId" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\Engine\Classes\AI\Navigation\NavLinkDefinition.h 40
Error (active) E0145 member "FRayTracingGeometry::NonSharedVertexBuffers" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\RenderCore\Public\RenderResource.h 665
Error (active) E0145 member "FShaderParameterBindings::kInvalidBufferIndex" may not be initialized UE4 L:\UE4.27.2-Source\Engine\Source\Runtime\RenderCore\Public\Shader.h 601
Error C4273 'FBodyInstanceCore::FBodyInstanceCore': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodyInstanceCore.cpp 7
Error C4273 'FBodyInstanceCore::ShouldInstanceSimulatingPhysics': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodyInstanceCore.cpp 18
Error C4273 'LexToString': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodySetupCore.cpp 12
Error C4273 'LexToString': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodySetupCore.cpp 21
Error C4273 'LexToString': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodySetupCore.cpp 30
Error C4273 'UBodySetupCore::UBodySetupCore': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodySetupCore.cpp 39
Error C4273 'UBodySetupCore::GetCollisionTraceFlag': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\BodySetupCore.cpp 45
Error C4273 'OnUpdatePhysXMaterial': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\ChaosEngineInterface.cpp 12
Error C2491 'FPhysicsDelegatesCore::OnUpdatePhysXMaterial': definition of dllimport static data member not allowed ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\ChaosEngineInterface.cpp 12
Error C4273 'LogPhysicsCore': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysicsCore.cpp 5
Error C4273 'GetPhysXCookingModule': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXPublicCore.cpp 92
Error C4273 'GPhysXAllocator': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXSupportCore.cpp 11
Error C4273 'GPhysXSDK': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXSupportCore.cpp 13
Error C4273 'GPhysXPendingKillMaterial': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXSupportCore.cpp 15
Error C4273 'GApexSDK': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXSupportCore.cpp 18
Error C4273 'GApexModuleLegacy': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXSupportCore.cpp 21
Error C4273 'GApexModuleClothing': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Source\Runtime\PhysicsCore\Private\PhysXSupportCore.cpp 25
Error C4273 'FBodyInstanceCore::StaticStruct': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\BodyInstanceCore.gen.cpp 20
Error C4273 'StaticStruct': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\BodyInstanceCore.gen.cpp 30
Error C4273 'UBodySetupCore::StaticRegisterNativesUBodySetupCore': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\BodySetupCore.gen.cpp 25
Error C4273 'UBodySetupCore::GetPrivateStaticClass': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\BodySetupCore.gen.cpp 137
Error C4273 'StaticClass': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\BodySetupCore.gen.cpp 139
Error C4273 'UBodySetupCore::UBodySetupCore': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\BodySetupCore.gen.cpp 143
Error C4273 'StaticEnum': inconsistent dll linkage ... L:\UE4.27.2-Source\Engine\Intermediate\Build\Win64\UE4Editor\Inc\PhysicsCore\ChaosEngineInterface.gen.cpp 225
I feel like I’ve forgotten to add something, somewhere. 