In order to enable PvE you will need to add the following line in the GameUserSettings.ini file in the [ServerSettings] section:
serverPVE=true
In order to disable damage between players (PvP) you will have to add the following line in the game.ini file in the [/Script/ShooterGame.ShooterGameMode] section:
bPvEDisableFriendlyFire=true