To edit the HTML template of your Blogger site.
1. Click the Template link which is present on the Left side of blogger menu
2. Choose Edit HTML option
3. Copy the container snippet
GTM Container Snippet something like this
<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-PQRSTUVWXYZ');</script>
In the snippet find the following characters 'dataLayer'?'&l='+l and change the ampersand symbol (&) to its HTML entities i.e (&).After changing ampersand symbol to its HTML entities then the character will look like this 'dataLayer'?'&l='+l
After changing & symbol to its corresponding HTML entity the entire container snippet will look like this
<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-PQRSTUVWXYZ');</script>
This needs to be done because the Blogger using strict encoding and non-HTML entities will cause errors in the template.