files I tested on

BuildInDungeon.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using BepInEx;
using HarmonyLib;
using UnityEngine;

namespace Valheim_BuildInDungeon
{
	[BepInPlugin(GUID, Name, Version)]
	[BepInProcess("valheim.exe")]
	[HarmonyPatch]
	public class ChangePlacePiece : BaseUnityPlugin
	{
		private const string GUID = "valheim.lottievixen.build_in_dungeons";
		private const string Name = "Build In Dungeons";
		private const string Version = "1.0.0.0";

        void Awake()
        {
            var harmony = new Harmony(GUID);
            harmony.PatchAll();
        }

		[HarmonyPostfix]
		[HarmonyPatch(typeof(Piece), "Awake")]
		static void Placement_Patch(Piece __instance)
		{
			__instance.m_allowedInDungeons = true;
		}
	}
}

DrawDebugSphere.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class DrawDebugSphere : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        
    }

    // Update is called once per frame
    void Update()
    {
        
    }

    private void OnDrawGizmos()
    {
        Gizmos.color = new Color(1, 0, 0, 0.5F);
        Gizmos.DrawWireSphere(transform.position, 0.2f);
    }
}