The game is installed via Steam on Ubuntu 16.04. 

When I try to launch a screeps server on localhost, its backend crushes with the following error message:

Loading mods from "/home/lunin/.steam/steam/steamapps/common/Screeps/server/mods.json"
Connecting to storage
Starting CLI server
STEAM_KEY environment variable is not found, trying to connect to local Steam client
Error: Steam client is not running
at Object.startServer (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/@screeps/backend/lib/game/server.js:104:19)
at common.storage._connect.then.then (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/@screeps/backend/lib/index.js:18:28)
at _fulfilled (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/q/q.js:834:54)
at self.promiseDispatch.done (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/q/q.js:863:30)
at Promise.promise.promiseDispatch (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/q/q.js:796:13)
at /home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/q/q.js:604:44
at runSingle (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/q/q.js:137:13)
at flush (/home/lunin/.steam/steam/steamapps/common/Screeps/server/node_modules/q/q.js:125:13)
at _combinedTickCallback (internal/process/next_tick.js:67:7)
at process._tickCallback (internal/process/next_tick.js:98:9)
Loading mods from "/home/lunin/.steam/steam/steamapps/common/Screeps/server/mods.json"
Connecting to storage

However, Steam client is up (obviously), as the game was launched using it. I also tried to manually set the STEAM_KEY environment variable, but it didn't help.