Firstly, check if the Google Tag Manager script is inserted in your website's code.
Here's how you can do this:
Go to your web page, click the mouse's right button, and select "View page source."
When you see the web page code, click on Ctrl+F and enter "www.googletagmanager.com/gtm.js" in the search line.
If you didn't find it, go to your GTM account and generate the script for your web page. Once you've done it, you can copy it and insert it into your web page code. Here're some tips for installing your Google Tag Manager code to the web page from Google.
If you've already inserted the GTM script on your web page, go to the next step and implement the number insertion script by creating a tag for it.