Can't compile on ue and have : was modified or is new

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;
};