nvidia-smi: command not found, but GPU works fine

Solution 1:

nvidia-smi is installed via nvidia-utils, as shown here:

$ sudo apt-get install nvidia-smi     
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package nvidia-smi is a virtual package provided by:
  nvidia-utils-440 440.44-0ubuntu0~0.18.04.1
  nvidia-utils-435 435.21-0ubuntu0.18.04.2
  nvidia-utils-430 430.64-0ubuntu0~gpu18.04.1
  nvidia-utils-415 415.27-0ubuntu0~gpu18.04.2
  nvidia-utils-410 410.104-0ubuntu0~18.04.1
  nvidia-utils-390 390.129-0ubuntu0~gpu18.04.1
You should explicitly select one to install.

E: Package 'nvidia-smi' has no installation candidate

So, in your case you'll need to install nvidia-utils-440.