Open the ServerGrid.Json file in a text editor and search for "bDontUseClaimFlags" (without the quotes). You should see something like this.
"OverrideShooterGameModeDefaultGameIni": {
"bDontUseClaimFlags": "1",
"NoClaimFlagDecayPeriodMultiplier": "4"
},
To allow claim flags, remove the following including all quotes and punctuation. Do not remove the curly braces or anything else.
"bDontUseClaimFlags": "1",
The second value controls decay of buildings if not inside a claimed area. If you remove that then I believe you can build anywhere without needing to claim to avoid decay.
"NoClaimFlagDecayPeriodMultiplier": "4"