How to make the turret not placeable on walls?

So I am working on a little game where you can place turrets and deffend points and I have this small issue. I can place turrets everywhere, even on walls and I dont want that. How can I make the turrets not placeable on walls ?

Here is the script:

     public GameObject blueprintGreen;
     public GameObject blueprintRed;
     public GameObject turret;
 
     private GameObject moveableBlue;
     private bool blueprintOn;
 
     [SerializeField]
     private Vector3 offset;
 
     [SerializeField]
     private float rotation;
 
     [SerializeField]
     private LayerMask mask;
 
     // Start is called before the first frame update
     void Start()
     {
         blueprintOn = false;
     }
 
     // Update is called once per frame
     void Update()
     {
         if(Input.GetKeyDown(KeyCode.Q) && blueprintOn == false)
         {
             BluePrint();
         }
         if (Input.GetKeyDown(KeyCode.Mouse0) && blueprintOn == true)
         {
             BuildTurret();
             Destroy(moveableBlue);
             blueprintOn = false;
             rotation = 0f;
         }
 
         if(Input.GetKey(KeyCode.Z) && blueprintOn == true)
         {
             rotation -= 2.5f;
         }
         if(Input.GetKey(KeyCode.X) && blueprintOn == true)
         {
             rotation += 2.5f;
         }
 
         if (blueprintOn)
         {
             MoveBluePrint();
         }
     }
 
     void BluePrint()
     {
         moveableBlue = Instantiate(blueprintGreen);
         blueprintOn = true;
     }
 
     void MoveBluePrint()
     {
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
         RaycastHit rayCast;
         if(Physics.Raycast(ray, out rayCast, mask))
         {
             moveableBlue.transform.position = rayCast.point + offset;
             //moveableBlue.transform.rotation = Quaternion.FromToRotation(Vector3.up, rayCast.normal);
             if (blueprintOn == true) moveableBlue.transform.rotation = Quaternion.Euler(0f, rotation, 0f);
         }
     }
 
     void BuildTurret()
     {
         Instantiate(turret, moveableBlue.transform.position, moveableBlue.transform.rotation);
     }