Isaac, here is the code.
But this won't be installed until we have the site built.

<!-- Google Tag Manager -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-MRX4ZJN');</script>
<!-- End Google Tag Manager -->

<!-- Google Tag Manager (noscript) -->
<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-MRX4ZJN"
height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
<!-- End Google Tag Manager (noscript) -->

Hector  - Got it.

Isaac  
Now that the site is ready, can you please add this GTM code to it please?
Thanks. 

Isaac, while you are at it, can you also please add the Google Analytics code?
----
<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-152797224-1"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-152797224-1');
</script>
----
Thanks. 

Hector - ADDED

Thank you Isaac . 
One thing, when I try to login using 
coolaesthetics.com/wp-login-php
I am getting a 404 error. 
What url are you using to login for this client?

Hector  

Below one - 
https://coolaesthetics.com/wp-login.php

Thank you Isaac, 
That worked for me. 
I have also verified both GTM and Google Analytics accounts are added and match.