Hi Jihun.
There are a couple problems here. The first is that you are setting both an Edit and a Visibility UProperty for the variable. You are only able to have one or the other in the UProperty list (just an FYI, the EditDefaultsOnly flag includes the VisibleDefaultsOnly tag).
The second issue is that the TSubobjectPtr variable you are implementing the UProperty flags for (if I found the same tutorial that you were looking at) cannot be set to editable, so you will only be able to use the VisibleDefaultsOnly flag.
Try using the following line:
UPROPERTY(VisibleDefaultsOnly, Category = Powerup)
That will resolve the error message you were seeing.
Have a great day.