VBOX GuestAdditions Error with ClearLinux

Installed ClearLinux in a VirtualBox. All runs fine.
But when I want to install the GuestAdditions I get the error:

Kernel headers not found for target kernel 5.4.75-75.lts2019.

How can I install this headers ?!?
Also system seems to be up2date. No updates can be installed. So how can it be the headers are missing ?!?