Rotate Object while placing it in Gameworld

Hi guys. I have a problem, i tried to create a simple "place object from inventory into gameworld" script. placing works fine but i have some problems with the rotation. the object should rotate slowly around itself while pressing A. here is the script:

 using UnityEngine;
 using System.Collections;
 
 public class BuildingPlacement : MonoBehaviour {
 
     public float scrollSensitivity;
         
     private PlaceableBuilding placeableBuilding;
     private Transform  currentBuilding;
     private bool hasPlaced;
     
     public LayerMask buildingsMask;
     
     private PlaceableBuilding placeableBuildingOld;
     
     // Update is called once per frame
     void Update () {
     
         Vector3 m = Input.mousePosition;
         m = new Vector3(m.x,m.y,transform.position.y);
         Vector3 p = camera.ScreenToWorldPoint(m);
         
         if (currentBuilding != null && !hasPlaced){
             //rotate object
             if (Input.GetKey(KeyCode.A))
             transform.Rotate(Vector3.down * speed * Time.deltaTime);
             
             currentBuilding.position = new Vector3(p.x,0,p.z);
             
             if (Input.GetMouseButtonDown(0)) {
                 if (IsLegalPosition()) {
                     hasPlaced = true;
                 }
             }
         }
         else {
             if (Input.GetMouseButtonDown(0)) {
                 RaycastHit hit = new RaycastHit();
                 Ray ray = new Ray(new Vector3(p.x,8,p.z), Vector3.down);
                 if (Physics.Raycast(ray, out hit,Mathf.Infinity,buildingsMask)) {
                     if (placeableBuildingOld != null) {
                         placeableBuildingOld.SetSelected(false);
                     }
                     hit.collider.gameObject.GetComponent<PlaceableBuilding>().SetSelected(true);
                     placeableBuildingOld = hit.collider.gameObject.GetComponent<PlaceableBuilding>();
                 }
                 else {
                     if (placeableBuildingOld !=null) {
                         placeableBuildingOld.SetSelected(false);
                     }
                 }
             }
         }
     }
 
     bool IsLegalPosition() {
         if (placeableBuilding.colliders.Count > 0) {
             return false;
         }
         return true;
     }
     
     public void SetItem(GameObject b) {
         hasPlaced = false;
         currentBuilding = ((GameObject)Instantiate(b)).transform;
         placeableBuilding = currentBuilding.GetComponent<PlaceableBuilding>();
     }
 }

The wrong line is on line 25 and marked with //rotate object. Unity says: - The name "speed" does not exist in the current contex - The best overloaded method match for UnityEngine.Transform.Rotate(UnityEngine.Vector3)' has some invalid arguments - Argument #1' cannot convert object' expression to type UnityEngine.Vector3'

yeah. and to be honest.. i have no idea whats wrong. could you help me please? thanks!