Thanks, Andrew. Your hint is very helpful. I thought the warning means AG couldn't specify the pins I draw in layout. Now instead of previously creating pin shapes, I put text labels on Metal1, and this way cleans out the warning msg.
Besides, I have new question if you don't mind. After finish running the "Abstract" step, I checked the abstracted layout and noticed that there is only M1 layer left while the rest is all gone. But the layout from its last step "Extract" doesn't lose anything. There is no warning msg regarding to this case, and I think all layers should be defined already. Thanks again.