It is a requirement that developers install the chrome plugin to validate the GTM code:
It is recommended to set chrome plugin to “Detailed” mode, rather than “Basic” mode:
Clicking on the “not working” shows details:
Tip: JSON validator is really good for checking the dataLayer
http://jsonlint.com/ for example
Note: jsonlint.com does not like HTML comments, and single-quotes need to be changed to double-quotes in order to validate.
Also JSON formatter for chrome can also be used to make JSON easier to read.