a is set to 0 every time you call previousVisitors(). You need to declare the variable outside of the function like the others.

let count = 0;
let entryEl = document.getElementById("entry-el");
let leftEl = document.getElementById("left-el");
let countEl = document.getElementById("count-el");
let previousEl = document.getElementById("previous-el");
let a = 0;

function peopleInShop() {
  //count += 1
  count++;
  countEl.textContent = count;
  if (count <= 5) {
    window.alert("Welcome");
  }
  if (count >= 6 && count !== 0) {
    window.alert("You can not Entry to the shop now! Wait till someone left the shop!");
  }
  if (count === 6) {
    //count -= 1;
    count--;
    countEl.textContent = 5;
  } else {
    //pass
  }

}

function left() {
  count--;
  if (count === -1) {
    count++;
  }
  countEl.textContent = count;

}

function previousVisitors() {
  a++;
  previousEl.textContent = a;
}

<h1> People in the gallery:</h1>
<h2 id="count-el"> 0</h2>
<button id="entry-el" onclick="peopleInShop() ; previousVisitors()">Entry</button>
<button id="left-el" onclick="left()">Left</button>
<p> Previous Entries: </p>
<h3 id="previous-el">0</h3>
<br>
<p><u>Ther avarage quantity of people in the shop can not be higher than five persons!</u></p>

Also, pass is declared in Python but not JS. You can just leave those brackets empty. Also, in JS (and many other languages), += 1 and -= 1 can be shortened to ++ and -- respectively. And if you know that two variables will always be of the same type, then you can use === instead of == (JS only). === is faster since == converts data types in JS. Also, please end every line with a semicolon. It is not required in JS, but it is a standard, unlike Python.

Hi,

First you have pass in your code that is undefined

So js code stop on the line 22

If you haven't any to statement on fault just remove else statement

In previousVisitors function you have declare a variable in the function

So a always = 0

And you use a = a + 1

So previousVistors will always be 1

To Solve declare a variable in global scoop not in function

like this

let a = 0;
function previousVisitors() {
  a = a + 1;
  previousEl.textContent = a;
}

let a = 0    
function previousVisitors() {
  a +=1
  previousEl.textContent = a
}

In your last function: Define "a" variable, And update the value when calling this function (When pressing the Button),

Call the function like That:

<button id="left-el" onclick="left();previousVisitors()">Left</button>