NVIDIA-SMI has failed because it couldn't communicate with the NVIDIA driver. Make sure that the latest NVIDIA driver is installed and running

If your nvidia-smi failed to communicate but you've installed the driver so many times, check prime-select.

1. Run prime-select query to get all possible options. You should see at least nvidia | intel.
2. Choose prime-select nvidia.
3. If it says nvidia is already selected, select a different one, e.g. prime-select intel, then switch back to nvidia prime-select nvidia
4. Reboot and check nvidia-smi.

You may want to install cuda toolkit. Using the following command to install it.

sudo apt install nvidia-cuda-toolkit

Once the installation is done, reboot the machine. nvidia-smi should work.

I disabled the Secure Boot and it worked pretty fine.

@rod-smith aswered another question more specific explaining how to do it, basically is a setup config, but he also write a good article about how to do that in here.

since I cannot comment on @Rodolfo's answer above (not enough reputation), I am adding a new answer.

On my machine I had to configure Secure Boot accordingly to my OS. I have an ASUS mainboard running Ubuntu 18.04 and tried to install NVIDIA CUDA 10.1 Update 2 with the packaged NVIDIA driver. I faced the same issue as described above. As it turned out, Secure Boot was set to Windows UEFI mode. Changing it to Other OS fixed it for me.

The solution by Markus lead me to a better solution. So it has to do with Secure Boot, but it is not necessary to deactivate.

To fix the problem, just do 3 steps: Deactivate the Nvidia driver by choosing X.Org with the Additional Drivers tool, reboot, then activate the Nvidia driver, reboot and enroll the key in Secure Boot.

Usually when you activate the Nvidia driver with the Additional Drivers tool, you are asked for a (new) password for Secure Boot. After reboot, the PC jumps into Secure Boot settings and you are asked to enroll a new MOK key, which must be confirmed with that same password. Afterwards, the driver will get access to the Nvidia card and will work.