Kernel headers unavailable for app build (linux-4.4.190-x86_64)

I've been messing around with the ReadyNAS OS6 virtual machine and wanted to see if I could make it recognize a USB device (mainly for encrypted volume recovery). That requires compiling and installing the VBox guest additions. So, I followed the normal process for doing that on a headless Debian system, and got an error when apt-get tried to install the kernel headers: I found this post Linux-Headers in the forum and did what WhoCares_  suggested. But when I executed make clean && make && make install && make modules_install, I got this:

HOSTCC scripts/basic/fixdep
HOSTCC scripts/kconfig/conf.o
HOSTCC scripts/kconfig/zconf.tab.o
HOSTLD scripts/kconfig/conf
scripts/kconfig/conf --silentoldconfig Kconfig
***
*** Configuration file ".config" not found!
***
*** Please run some configurator (e.g. "make oldconfig" or
*** "make menuconfig" or "make xconfig").
***
scripts/kconfig/Makefile:37: recipe for target 'silentoldconfig' failed
make[2]: *** [silentoldconfig] Error 1
Makefile:540: recipe for target 'silentoldconfig' failed
make[1]: *** [silentoldconfig] Error 2
SYSTBL arch/x86/entry/syscalls/../../include/generated/asm/syscalls_32.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_32.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_64.h
SYSHDR arch/x86/entry/syscalls/../../include/generated/uapi/asm/unistd_x32.h
HOSTCC arch/x86/tools/relocs_32.o
HOSTCC arch/x86/tools/relocs_64.o
HOSTCC arch/x86/tools/relocs_common.o
HOSTLD arch/x86/tools/relocs
make: *** No rule to make target 'include/config/auto.conf', needed by 'include/config/kernel.release'. Stop.

I tried to install the guest additions anyway, in case enough worked, and got this (in addition to some things that did seem to work):

VirtualBox Guest Additions: Kernel headers not found for target kernel
4.4.190.x86_64.1. Please install them and execute
/sbin/rcvboxadd setup
modprobe vboxguest failed

So, no dice. I'm no Linux programmer, but it appears clear to me that something is missing or misconfigured in the linux-4.4.190-x86_64 directory of ReadyNASOS_V6.10.3_WW_src.zip. And from what I've read here, it's a regular occurrance that something is wrong or missing whenever an OS update occurs.  I guess it's no wonder hardly anybody even tries to develop apps for the ReadyNAS any longer when Netgear doesn't provide the proper (and as I understand it required by the GPL license) sources.

I guess I should also wonder why, apt-get install linux-headers-generic linux-headers-$(uname -r) does not work and the whole kernel build is necessary at all.  That command gives me this:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package linux-headers-generic is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'linux-headers-generic' has no installation candidate
E: Unable to locate package linux-headers-4.4.190.x86_64.1
E: Couldn't find any package by glob 'linux-headers-4.4.190.x86_64.1'
E: Couldn't find any package by regex 'linux-headers-4.4.190.x86_64.1'