As I understand the error occurs because I do not have any rules assigned. This does not affect the gameplay, just want you to take into account.

"RuleSets": [
    {...},
    {
      "name": "pvp",
      "enabled": true,
      "defaultAllowDamage": true,
      "flags": "",
      "rules": []
    }
  ]​

that did not cause any error for me. post the config

{
  "Config Version": "2.0.9",
  "Default RuleSet": "default",
  "Configuration Options": {
    "handleDamage": true,
    "useZones": false,
    "Trace To Player Console": false,
    "Trace To Server Console": false,
    "Maximum Distance From Player To Trace": 0.0,
    "Prevent Water From Extinguishing BaseOven": false,
    "Prevent Players From Being Marked Hostile": false
  },
  "Mappings": {
    "default": "default",
    "pvp_purge": "pvp_purge"
  },
  "Schedule": {
    "enabled": false,
    "useRealtime": true,
    "broadcast": true,
    "entries": [
      "4.18:00 pvp_purge <size=16>You are allowed to damage each other!</size>",
      "5.10:00 default"
    ]
  },
  "RuleSets": [
    {
      "name": "default",
      "enabled": true,
      "defaultAllowDamage": false,
      "flags": "HumanNPCDamage, PlayerSamSitesIgnorePlayers, TurretsIgnorePlayers, SelfDamage, AuthorizedDamage, TwigDamage, NoHeliDamageQuarry",
      "rules": [
        "anything can hurt resources",
        "anything can hurt helicopters",
        "players cannot hurt players",
        "fire cannot hurt buildingblocks",
        "barricades can hurt players",
        "highwalls can hurt players",
        "fire can hurt players",
        "ch47 can hurt players",
        "patroolheli can hurt players",
        "npcs can hurt players",
        "guards can hurt players",
        "traps cannot hurt anything",
        "bradleyAPC can hurt players",
        "players can hurt bradleyAPC",
        "players can hurt npcs",
        "animals can hurt players",
        "players can hurt animals"
      ]
    },
    {
      "name": "pvp_purge",
      "enabled": true,
      "defaultAllowDamage": true,
      "flags": "",
      "rules": []
    }
  ],
  "Entity Groups": [
    {
      "name": "animals",
      "members": "RidableHorse, Horse, Bear, Boar, Polarbear, Wolf, Zombie",
      "exclusions": ""
    },
    {
      "name": "barricades",
      "members": "Barricade, icewall, GraveYardFence",
      "exclusions": ""
    },
    {
      "name": "dispensers",
      "members": "BaseCorpse, HelicopterDebris, PlayerCorpse, NPCPlayerCorpse, HorseCorpse",
      "exclusions": ""
    },
    {
      "name": "fire",
      "members": "FireBall, FlameExplosive, FlameThrower, BaseOven, FlameTurret, rocket_heli_napalm, napalm, oilfireball2, oilfireballsmall",
      "exclusions": ""
    },
    {
      "name": "guards",
      "members": "bandit_guard, scientistpeacekeeper, sentry.scientist.static",
      "exclusions": ""
    },
    {
      "name": "patroolheli",
      "members": "BaseHelicopter",
      "exclusions": ""
    },
    {
      "name": "highwalls",
      "members": "SimpleBuildingBlock, wall.external.high.ice, gates.external.high.stone, gates.external.high.wood",
      "exclusions": ""
    },
    {
      "name": "buildingblocks",
      "members": "BuildingBlock",
      "exclusions": ""
    },
    {
      "name": "cars",
      "members": "BasicCar, ModularCar, BaseModularVehicle, BaseVehicleModule, VehicleModuleEngine, VehicleModuleSeating, VehicleModuleStorage, VehicleModuleTaxi, ModularCarSeat",
      "exclusions": ""
    },
    {
      "name": "helicopters",
      "members": "MiniCopter, ScrapTransportHelicopter",
      "exclusions": ""
    },
    {
      "name": "ch47",
      "members": "ch47.entity",
      "exclusions": ""
    },
    {
      "name": "npcs",
      "members": "ch47scientists.entity, HumanNPC, NPCPlayer, ScientistNPC, TunnelDweller, SimpleShark, UnderwaterDweller, Zombie, ZombieNPC",
      "exclusions": ""
    },
    {
      "name": "bradleyAPC",
      "members": "BradleyAPC",
      "exclusions": ""
    },
    {
      "name": "players",
      "members": "BasePlayer, FrankensteinPet",
      "exclusions": ""
    },
    {
      "name": "resources",
      "members": "ResourceEntity, TreeEntity, OreResourceEntity, LootContainer",
      "exclusions": "hobobarrel.deployed"
    },
    {
      "name": "samsites",
      "members": "sam_site_turret_deployed",
      "exclusions": "sam_static"
    },
    {
      "name": "traps",
      "members": "AutoTurret, BearTrap, FlameTurret, Landmine, GunTrap, ReactiveTarget, TeslaCoil, spikes.floor",
      "exclusions": ""
    },
    {
      "name": "junkyard",
      "members": "magnetcrane.entity, carshredder.entity",
      "exclusions": ""
    }
  ],
  "Allow Killing Sleepers": false,
  "Allow Killing Sleepers (Ally Only)": false,
  "Ignore Firework Damage": true
}

schedule is not enabled, and there is no error still

> o.reload TruePVE
TruePVE was compiled successfully in 1617ms
Unloaded plugin TruePVE v2.0.9 by nivex
Loaded plugin TruePVE v2.0.9 by nivex

when does the error occur?