Why does CreateSession fail with OnlineSubsystem null?

OnlineSubsystem plugin not enabled, or no DefaultPlatformService set in DefaultEngine.ini. Enable plugin (Plugins window), set [OnlineSubsystem] DefaultPlatformService=NULL (testing) or =Steam/EOS for production.

NULL subsystem for testing?

Yes — NULL subsystem provides LAN-only sessions, useful for local dev. Production must use real platform (Steam, EOS, EOSPlus, GOG, Live).

V2 (Online Services API) is the new way; V1 OnlineSubsystem still works. UE5.4+ recommends V2 for new code; pick by ecosystem maturity.

Fix: Unreal CreateSession Fails OnlineSubsystem Null

Quick answer: Plugins → enable OnlineSubsystem and OnlineSubsystemNull (or Steam). DefaultEngine.ini: [OnlineSubsystem] DefaultPlatformService=NULL.

CreateSession returns false. Logs show OnlineSubsystem null. Plugin disabled or no platform service configured.

The Fix

# DefaultEngine.ini
[OnlineSubsystem]
DefaultPlatformService=NULL

[OnlineSubsystemNull]
bEnabled=true

Edit → Plugins → search OnlineSubsystem → Enable both Base and NULL. Restart editor.

For Steam Production

[OnlineSubsystem]
DefaultPlatformService=Steam

[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=480

[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="OnlineSubsystemSteam.SteamNetConnection"

Verifying

Log shows OnlineSubsystem NULL initialized. CreateSession succeeds. LAN games discoverable.

“Plugin enabled. Service set. Sessions create.”

Related Issues

For replication scope, see replication scope. For BP timer server-only, see timer server.

Plugin on. INI set. Session creates.