https://aur.archlinux.org/packages/opensnitch-ebpf-module?O=0#comment-965841
Manjaro does not create the /usr/src/linux symlink, which should point to the current kernel headers and is required for this package to build properly.
Manjaro has several kernels in the format linuxMajorMinor e.g. linux66 or linux68 and any number of kernels can installed.
To avoid confusion and possible errors when the symlink point to linux68 and booted kernel is linux66 the symlink does not exist.