Only able to save inside the editor problem

I just bought a package and it says it can save and it can but when I try to build it it comes up with a error about saving. How do I make it save some were else so that this doesn't happen. heres the error

here is the script

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 
 public class ModelEditorGUI : MonoBehaviour {
 
     public enum GuiState{
         Edit,
         Save,
         Load
     }
     public static GuiState currentGuiState = GuiState.Edit;
 
     float sW = Screen.width;
     float sH = Screen.height;
 
     Rect colorRect = new Rect(Screen.width / 150, Screen.height / 100, Screen.width / 8, Screen.height / 8);
 
     public static byte r, g, b;
 
     public static Color32 currentColor;
 
     public GUISkin skin;
 
     public static int buildMode = 1;
     public static int colorMode = 0;
 
     public GUITexture crosshair;
 
     string modelName = "";
 
     //Icons
 
     public Texture2D icon_colorPicker;
     public Texture2D icon_colorChanger;
 
     void Start () {
 
         CustomMouseLook.canLook = true;
         Screen.lockCursor = true;
         ModelEditorPlayer.canBuild = true;
 
     }
 
     void Update () {
         currentColor = new Color32(r,g,b,(byte)(colorMode == 0 ? 255 : 200));
         if(Input.GetKey(KeyCode.LeftControl)){
             CustomMouseLook.canLook = false;
             Screen.lockCursor = false;
             ModelEditorPlayer.canBuild = false;
         }
         else if (currentGuiState == GuiState.Edit){
             CustomMouseLook.canLook = true;
             Screen.lockCursor = true;
             ModelEditorPlayer.canBuild = true;
         }
 
 
     }
 
     void OnGUI () {
         GUI.skin = skin;
 
         //Color Area
 
         if(currentGuiState == GuiState.Edit){
 
             ModelEditorPlayer.canBuild = true;
             crosshair.enabled = true;
 
             GUI.Box(colorRect, "");
             GUILayout.BeginArea(colorRect);
 
             GUI.skin.label.alignment = TextAnchor.MiddleCenter;
             GUILayout.Label("Color");
             r = (byte)GUILayout.HorizontalSlider((int)r, 1, 255);
             g = (byte)GUILayout.HorizontalSlider((int)g, 1, 255);
             b = (byte)GUILayout.HorizontalSlider((int)b, 1, 255);
             GUILayout.BeginHorizontal();
             if(GUILayout.Button("Toggle Mode", GUILayout.Width(sW / 18))) buildMode = buildMode == 0 ? 1 : 0;
             if(GUILayout.Button("Toggle Color", GUILayout.Width(sW / 18))) colorMode = colorMode == 0 ? 1 : 0;
             GUILayout.EndHorizontal();
             GUILayout.EndArea();
         }
         else if(currentGuiState == GuiState.Save){
             ModelEditorPlayer.canBuild = false;
             crosshair.enabled = false;
             CustomMouseLook.canLook = false;
             Screen.lockCursor = false;
 
 
             GUI.Box(new Rect(Screen.width/2-Screen.width/10, Screen.height/2-Screen.height/12, Screen.width/5, Screen.height/6), "Save Map");
             GUI.BeginGroup(new Rect(Screen.width/2-Screen.width/10, Screen.height/2-Screen.height/12, Screen.width/5, Screen.height/6));
             
             GUI.Label(new Rect(10, 30, 100, 20), "Model Name:");
             modelName = GUI.TextField(new Rect(80, 30, 130, 20), modelName);
             
             if(GUI.Button(new Rect(10, 70, 200, 25), "Save Model as File")){
                 ModelSerializer.SaveModel(modelName);
                 currentGuiState = GuiState.Edit;
             }
             if(GUI.Button(new Rect(10, 95, 200, 25), "Save Model as Mesh")){
 
 
                 AssetDatabase.CreateAsset(ModelSerializer.ModelToMesh(), "Assets/Models/Mesh/" + modelName + ".asset");
 
                 currentGuiState = GuiState.Edit;
             }
             
             GUI.EndGroup();
         }
         if(currentGuiState == GuiState.Load) {
             
             ModelEditorPlayer.canBuild = false;
             crosshair.enabled = false;
             CustomMouseLook.canLook = false;
             Screen.lockCursor = false;
             
             GUI.Box(new Rect(Screen.width/2-Screen.width/10, Screen.height/2-Screen.height/12, Screen.width/5, Screen.height/6), "Load Map");
             GUI.BeginGroup(new Rect(Screen.width/2-Screen.width/10, Screen.height/2-Screen.height/12, Screen.width/5, Screen.height/6));
             
             GUI.Label(new Rect(10, 30, 100, 20), "Model Name:");
             modelName = GUI.TextField(new Rect(80, 30, 130, 20), modelName);
             
             if(GUI.Button(new Rect(10, 70, 200, 25), "Load Model")){
                 StartCoroutine(ModelSerializer.LoadModel(modelName));
                 currentGuiState = GuiState.Edit;
             }
             
             GUI.EndGroup();
             
         }
 
     }
     
 
 }