URL contains no Google Tag Manager code
This means that the URL in question does not contain the Google Tag Manager snippet.
Why is this important?
Google Tag Manager (GTM) is user-friendly solution to managing tracking and analytics tags, for example Adwords Conversion Tracking and Google Analytics.
This Hint merely exists to highlight the absence of such a tag.
What does the Hint check?
Examples that trigger this Hint
The Hint would trigger for any URL which does NOT contain the following in the <head>:
AND does NOT contain the following in the <body>:
Why is this Hint marked 'Advisory'?
Most sites on the internet do not use GTM, so unless a site IS using GTM, you would actually expect this Hint to trigger for all the URLs.
This Hint becomes useful when you audit a site that does actively use GTM, as it will highlight the URLs for which the GTM code is not present, allowing you to check and potentially fix this.