Thanks for your reply. That was my first guess. But I checked the TGA folder first when hooking the textures and could not found a shared texture file so I didn't check the PNG folder very carefully, and kind missed the props_base_color.png in it. I opened it now and found it right where it has always been. So, sorry for all the trouble.
I'm learning Unreal online due to the lockdown and school holiday and in no hurry. But most tutorials or videos are quite vague on some of the basic contents, so a little tips or something like that on the instruction page should be enough helping more inexperienced users like myself figure things out maybe?
Your respond truely helps me, I'm actually learning things. Thank you again.