How to apply speed boost to player on mobile game

The only problem i see with your code is the fact that your setting the players speed to 5 if he does not touch the screen. in fact he can only touch the screen every 4 seconds.that doesnt give alot of time to touch the screen, maybe less than a frame?

maybe give a little time buffer for the players ability to tap.

 using System.Collections;
  using System.Collections.Generic;
  using UnityEngine;
  
  public class PlayerMovement2 : MonoBehaviour {
  
      public float movementSpeed = 0; //Do not adjust
      public float boost = 5;
      public float maxSpeed = 10;
      public float Acceleration = 10;
      public float Deceleration = 10;
      public GameObject RaceTimer;
 
      //tip: not specifying a modifier will make it private by default.
      bool ActivatedBoost;
      Rigidbody myBody;
  
      // Use this for initialization
      void Awake () 
      {
         myBody = GetComponent<Rigidbody>();
      }
      void FixedUpdate () 
      {
         //maybe you should look into invokerepeating?
         StartCoroutine(Move());
         StartCoroutine(speedBoost());
      }
 
      void Update() {
         //you should only be checking for input in the update method.
         if (Input.touchCount > 0 && Input.GetTouch(0).phase == TouchPhase.Ended) {
             ActivatedBoost = true;
         } //using an else here will break the code as i am setting ActivatedBoost to false in the speedboost method
             
      }
      
      IEnumerator Move()
      {
         yield return new WaitForSeconds(4f);
 
         PlayerMove();
      }
  
          IEnumerator speedBoost()
          {
             //this will make a 4 second activation period
             yield return new WaitForSeconds(4f);
             if (ActivatedBoost)
             {
                 movementSpeed = movementSpeed + boost;
                 ActivatedBoost = false;
             } else {
                 movementSpeed = 5; //floats only need their f suffix when youre using periods (ie, 3.14f)
             }
      }
  
      void PlayerMove()
      {
         myBody.velocity = new Vector3(movementSpeed, 0, 0f);
      }
  
      private void OnTriggerEnter(Collider other)
      {
         if (other.CompareTag("Player"))
         {
            RaceTimer.SetActive(false);
         }
      }
 }

As a usual, I commented a few tip and points you may want to read. you can remove them when your done reading them.