Hello Yun-Kun, thank you for the information! I see you have a YouTube channel for Tutorials, you should do one on saving all Actors in a level. There are only 2 video for this on YouTube and they are for saving only 1 actor. I could test the video for you 
I setup the Blueprint Structure and added the Actor Class and Transform, and also added the array of Actor classes to the save file. Now when I press the “K” key to create the save file nothing happens. I am probably not injecting your code correctly into the save file, do you have any advice, see screenshot below.