Gentoo's Bugzilla –
Attachment 569076 Details for
Bug 680296app-emulation/virtualbox-modules-6.0.4 with >=sys-kernel/gentoo-sources-5.0.0 - .../work/vboxdrv/Makefile.include.header:143: *** Error: unable to find the headers of the Linux kernel to build against. Specify KERN_VER=<version> (...) and run Make again.