I was trying to run a Preview of Google Tag Manager for a site I was working on, but for some reason it simply wasn’t showing. Turning to the Console as always when something unexpected happens (or doesn’t happen), I saw an error message about my ‘Content Security Policy directive’ being violated.
Basically, a CSP is a directive that says what content is allowed on a site. For example, it might say that external scripts aren’t allowed. And in my case, it wasn’t allowing the debug mode of GTM to be delivered from the domain tagmanager.google.com/debug.
To get round this (if you don’t want to change your CSP) use a handy Chrome extension called Disable Content-Security Policy.