julienlg
First of all, check if there are any errors in the console (in the developer tools, F12
or CTRL + SHIFT + I
), or just errors in general. Your code might also be invalid or has a syntax error, but Flarum should check for syntax.
Google also requires some additional steps to verify that you are really the owner of the webpage. If you haven't completed them yet, it won't work.
If we don't know the exact issue, we can just give you general/most common fixes. I recommend you to reconfigure it step-by-step while checking Google's how-to. I don't have much experience with Google Analytics, so I can't help you much with that, but maybe someone will provide you more information with a hope for a fix 🙂
Google's tutorial:
https://support.google.com/analytics/answer/1008015?hl=en