X and Z axis question ?

Hey guys. So I am new to unity. bear with me. This is my code. I want the player to be set on position X and move on horizontal axis.. based on left and right inputs and in z direction / forward.. I have declared all the variables and included the header files. so this is my code..i am getting the following errors :

     private Vector3 playerPos = new Vector3 (0.25f, 0f, 2.23f);
 
 void Start () {

     player = GameObject.FindGameObjectWithTag ("Player");

     mainCamera.transform.localPosition = new Vector3 ( 0, 0, 0 );
     mainCamera.transform.localRotation = Quaternion.Euler (18, 180, 0);
 
 }
 
 // Update is called once per frame
 void Update () {

 //    player.transform.localPosition.x = posX;
     float moveAmount = Time.smoothDeltaTime * 10;

     if(mainCamera.transform.localPosition.z >= 0)
     {
         movementZ = Input.GetAxis("Vertical") * Vector3.forward * moveSpeed * Time.deltaTime;
         playerPos = new Vector3 (newX, 0f, movementZ);   // says this line has invalid arguments
                    // also says for the above line.. cannot convert vector 3 to float.. 
         player.transform.position = playerPos;
     }
     
 }

can you tell me what i am doing wrong.

thanks.