I am running a Debian 9 at kernel version 5.10.0-rc1-sev-es+. The directory /lib/modules/5.10.0-rc1-sev-es+/build (which I need for compiling kernel modules) does not seem to exists. However, when I run sudo apt-get install linux-headers-$(uname -r) it simply tells me:
linux-headers-5.10.0-rc1-sev-es is already the newest version (5.10.0-rc1-sev-es-1).. Searching for the package also does not show the header files for 5.10.0-rc1-sev-es+:
sudo apt-cache search linux-headers-5.
linux-headers-5.2.0+ - Linux kernel headers for 5.2.0+ on amd64
linux-headers-5.10.0-rc1-sev-es - Linux kernel headers for 5.10.0-rc1-sev-es on amd64
linux-headers-5.1.0+ - Linux kernel headers for 5.1.0+ on amd64