/lib/modules/4.15.0-1021-oem/build is in linux-headers-4.15.0-1021-oem 4.15.0-1021.24.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.15.0-1021-oem
/usr/src/linux-headers-4.15.0-1021-oem