Loading AOSP problem in Android Studio

Hello everyone! I am now studying AOSP for developing custom ROM. However, I am suffering many problems from loading source codes in Android Studio.

There are many files mark errors like this screen shot. Resource cannot be loaded. Whatever I expanded the xms, xmx value IDEA_HOME/bin/studio64.vmoption, remove unnecessary JDK classpath, the problem still the same.

Thank you for your help

I am using Android 5.1 with Nexus 5













Thank you for your help!