I installed Atom using Snap.
Then I tried to install Hydrogen as an addon for Atom.
This output appears -
apm install hydrogen
Command 'apm' is available in '/snap/bin/apm'
The command could not be located because '/snap/bin' is not included in the PATH environment variable.
apm: command not found
Open Atom and select Settings tab -> click the blue Install button located on the left side of the Settings pane. Under the Featured Packages section Hydrogen is the first package in the list because it is the most popular Atom package. Click the blue Install button to install it in Atom. Alternatively you can also search for Hydrogen in the search box in the Install Packages section of the Settings pane.
Like it says - apm is not in your PATH. Add it like so:
export PATH=$PATH:/snap/bin/apm
Put it in ~/.profile - to run at terminal startup if you wish