Game can be played despite "server under inspection" message

[pc2]

If you lauch the game and get "server under inspection" method - click on Main Menu.

Then select Option - and move Music and SFX sliders until you get the music
Then select Play Survival - Practice Challenge. The game will try to start but no zombies.
Press the steam menu button on left hand controller.
Select Main Menu - Play Survival - Practice Challenge and it should work this time.
If you have no sound - go back to Options and move the Music and SFX sliders a bit.

Hope this works for you - it does on several PCs I have tried on