First, don't issue any make command in
the kernel/ dir. This is a common mistake
for those who have kernel building experiences.
If you have do so, clean it by make distclean.
(or, more punctiliously, rm -rf kernel/ && repo sync kernel )
Second, make sure you have download the
whole source tree correctly.
Repeatedly repo sync, until no error occurs.
Then make a clean build again.
You may just do it by rm -rf out.