Follow back through the thread a little bit.
You will see that the appid is specified in DefaultEngine.ini
[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=123456789
It defaults to 480 which is Valve’s “test” app id, but you will need to get your own id before shipping. If this value is set, then the code will create a steam_appid.txt file at launch before initializing Valve’s APIs. If you have Steam warnings/errors in your logs, compare them to what is in the thread already and then post here and I’ll try to get you straightened out.