This question already has an answer here:
My environment is a little different. I updated from 14.04 to 16.04 to 18.04.
My desktop PC 18.04 has npm installed.
$ npm -version 3.5.2 # nodejs -v v8.10.0 # apt install npm
Reading package lists... Done
Building dependency tree
Reading state information... Done Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation:
The following packages have unmet dependencies: npm : Depends: node-gyp (>= 0.10.9) but it is not going to be installed E: Unable to correct problems, you have held broken packages.