nvidia-smi command not found Ubuntu 16.04

The solution that worked for me was to disable secure boot when rebooting after installing the NVIDIA drivers.

sudo apt purge nvidia-*
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt update
sudo apt install nvidia-381

Then reboot, in the menu choose "change secure boot options", put the password you previously chose and disable the secure boot.

Try updating the driver.

1. Add the PPA by running the following commands in terminal:

   sudo add-apt-repository ppa:graphics-drivers/ppa
   sudo apt-get update
   

2. Open Software & Updates from System Settings or directly from Dash, click on the Additional Drivers tab, select the driver you want to use, and click Apply changes.

3. After the driver is downloaded and installed, restart your system.