OK, i synaptic just search for nvidia, omit the 396.
What I am not seeing in the aptitude search is something I expected, although mine is 390, to wit:
i A nvidia-dkms-390 - NVIDIA DKMS package
i A nvidia-driver-390 - NVIDIA driver metapackage
i A nvidia-kernel-common-390 - Shared files used with the kernel module
i A nvidia-kernel-source-390 - NVIDIA kernel source package
i A nvidia-prime - Tools to enable NVIDIA's Prime
I would expect aptitude to return those things, since without them, the kernel module cannot be built.
Please try to use synpatic or apt-get to install nvidia-driver-396, since that should pull in the other packages, unless synaptic says it is installed and aptitude does not, in which case, there is a problem with apt-get, and that is involved.
I am running out of ideas, but before I did anything, I would now run
dpkg --configure -a'just in case the script that the distro uses to compile and install the module did not for some reason work.
then use synaptic to install or reinstall "nvidia-driver- 396" and see what happens.
Normally, I simply wait until a distro is installed and use apt-get or synaptic or aptitude to install things like this, so I can troubleshoot if something takes a bif of a wrong turn, but you will get this done, I am sorry is appears so tricky.
I have not done a source install from the nvidia website download in a while, but that always works, presuming you are comfortable working in the terminal without a graphic interface loaded, and build and install all the requisite stuff by command. You seem to know how to do this, so keep going.
best,
martin