'TextMesh[]' does not contain a definition for 'text' and no extention method 'text' accepting first argument type 'TextMesh[]' could be found

This is my code:
using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class SliderHandle : MonoBehaviour {

public Slider thisSlider;

// Use this for initialization
void Start () {
    thisSlider = GetComponent<Slider>();
}

// Update is called once per frame
void Update () {
    GetComponentsInChildren<TextMesh>().text = thisSlider.value.ToString();
}

}

However, there is an error under text showing the error message in the title, could someone please help me?

Mind the s at GetComponentsInChildren !

By the way, you should cache the component instead of retrieving it every frame :

 private Slider thisSlider;
 private TextMesh textMesh;
 

 // Use this for initialization
 void Start () {
     thisSlider = GetComponent<Slider>();
     textMesh = GetComponentInChildren<TextMesh>();

 }
 
 // Update is called once per frame
 void Update () {
     textMesh .text = thisSlider.value.ToString();
 }

You can’t just grab .text on an array of textmesh objects
you have to loop over each one individually.

foreach (var tm in GetComponentsInChildren<TextMesh>()) {
tm.text = thisSlider.value.ToString();
}

Also it is worth it to note that GetComponentsInChildren is an expensive call, and it is not worth it to call it in Update method which is called every single frame.
Best to get the textmeshes only once in the Start method:
So you would have something like this:

private TextMesh[] textMeshes;
public Slider thisSlider;

void Start() {
    textMeshes = GetComponentsInChildren<TextMesh>();
    thisSlider = GetComponent<Slider>();
}

void Update() {
    foreach (var tm in textMeshes) {
         tm.text = thisSlider.value.ToString();
    }
}