GridHighlight.cs

public class GridHighlight : MonoBehaviour
{

    public Transform tilePrefab;
    public Transform newTile;
    public Vector3 objectPos;
    Vector3 mousePos;
    private Grid grid;

    // Start is called before the first frame update
    void Start()
    {
        
    }

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

    private void Awake()
    {
        grid = FindObjectOfType<Grid>();
    }

    void OnMouseOver()
    {
        if (!(Input.mousePosition.x == mousePos.x && Input.mousePosition.y == mousePos.y))
        {
            if (newTile != null)
            {
                Destroy(newTile.gameObject);
            }
            mousePos = Input.mousePosition;
            mousePos.z = 70f;
            Vector3 objectPos = Camera.current.ScreenToWorldPoint(mousePos);
            objectPos.y = 1.05f;
            // newTile = Instantiate(tilePrefab, objectPos, Quaternion.Euler(0,0,0)) as Transform;

            RaycastHit hitInfo;
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

            if (Physics.Raycast(ray, out hitInfo))
            {
                PlaceTileNear(hitInfo.point);
            }

        }
    }

    private void PlaceTileNear(Vector3 clickPoint)
    {
        var finalPosition = grid.GetNearestPointOnGrid(clickPoint);
        newTile = Instantiate(tilePrefab, finalPosition, Quaternion.Euler(0, 0, 0)) as Transform;
    }
}