You need to put it below the public property.
When you have you header file
#pragma once
// includes here
// class definition here
UCLASS()
class MYPROJECT_API AMyActor : public AActor
{
GENERATED_BODY()
public:
// Sets default values for this actors properties
AMyActor();
/// needs to be below public declaration
UPROPERTY(EditAnywhere, BlueprintReadWrite)
TObjectPtr<UPhysicalMaterial> PhysMat;
};