I'm having similar issues with getting monuments to spawn on land. I am playing on the Last Train map if that matters.
I get this console error often. In this case, I tried to spawn in 'busStopGarage' with /spawnmonument busStopGarage in chat
Failed to call hook 'SpawnMonumentChatCommand' on plugin 'DynamicMonuments v1.0.1' (NullReferenceException: Object reference not set to an instance of an object.)
at Oxide.Plugins.DynamicMonuments+MonumentSpawner+PrefabSpawner.GetRotation (System.String monumentPreset, Oxide.Plugins.DynamicMonuments+MonumentSpawner+PrefabData prefabData) [0x0005b] in <28bdcaf11d384951897ca5404e9c7651>:0
at Oxide.Plugins.DynamicMonuments+MonumentSpawner+PrefabSpawner.SpawnInRandomPosition (Oxide.Plugins.DynamicMonuments+MonumentSpawnInfo spawningMonumentInfo) [0x00052] in <28bdcaf11d384951897ca5404e9c7651>:0
at Oxide.Plugins.DynamicMonuments+MonumentSpawner.TrySpawnMonument (Oxide.Plugins.DynamicMonuments+MonumentSpawnInfo spawningMonumentInfo) [0x00025] in <28bdcaf11d384951897ca5404e9c7651>:0
at Oxide.Plugins.DynamicMonuments.SpawnMonumentChatCommand (BasePlayer player, System.String command, System.String[] arg) [0x00033] in <28bdcaf11d384951897ca5404e9c7651>:0
at Oxide.Plugins.DynamicMonuments.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00c57] in <28bdcaf11d384951897ca5404e9c7651>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <9c80d821d00a44c9a24497c73ad2d20d>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000de] in <d646191a355d43a6b3ab36b7ee14c740>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <d646191a355d43a6b3ab36b7ee14c740>:0