Most games require an initial start-up while connected to the Steam Network before they will work in offline mode.If your game's status is "100% - Ready", but you receive the message "This game cannot be started in Offline Mode" when attempting to play offline, the Steam client and/or game files need to be updated.
Stored information is required for offline mode to function correctly.
To prevent this information from being lost, make sure that you manually close Steam before shutting your computer down. You'll also want to make sure that you don't have any interfering programs affecting Steam or its installation files.
If you’re performed these steps, Steam’s offline mode should now work without any problems until the next time you tell Steam to go online.
If you didn’t perform the above steps, Steam should prompt you to enable offline mode if it can’t connect to Steam’s servers.
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Offline Mode allows you to play games through Steam without having an active connection to the Steam Network.
This feature is useful when you have limited or no internet access.
Any Steam client feature that requires a connection will be unavailable while offline.
Also keep in mind that while in Offline Mode, Steam and your games will not receive important updates.
It's always a good idea to go online to update the Steam client as well as your games.
Unfortunately, this approach won’t work for most games – but it’s worth a try if Steam’s offline mode isn’t working for you.