GTM doesn't work properly

Can you elaborate more to understand the issue properly?
If you set the container ID properly in admin/config/system/google-tag, then it will load a script that contains the container ID and will be initiate d from files/google_tag/primary/google_tag.script.js. If you inspect and take the page source you can see the script if you configured the container ID properly.

I'm having the same problem. Everything appears to be configured properly, but Google Tag Script is not showing up in the page source.

I disabled the container on /admin/config/system/google-tag and then re-enabled it and it started working!