You can use Virtual Page Views, sending a "pageview" event to Google Analytics of a page that doesn't actually exist. You can do it this way:
ga('send', 'pageview', '/virtualpageview/nameofthepage')
This will make the pageview appears on your Google Analytics and you can use it to create Goals and/or Events.
The answer depends on (A) what you mean by "visitor behaviour" and (B) how much you are prepared to do in terms of adding to or drawing from what's available in Google Analytics 'out of the box'.
GA will track some visitor behaviour 'out of the box'. E.g. the pages the user visits, when they visited those pages, the source/medium of the session, and so on. ...
You likely don't have an error on your pages.
If that is the case you can select to ignore the diagnostic message in the interface (or possibly select it as resolved via the diagnostic message- you'll have to check that, brain is a bit hazy on it).
Based on your timezone settings in GA, sessions that are current at midnight will end and generate a new ...
If the production environment is in relation to the live site and has the default GTM container code installed on it, it doesnt need to be specified within GTM.
Every container is set up with a "Live" environment by default, which
always points to the container version that is currently published.
You don't need to define the Live environment.
You will need to install the GTM container code onto the applicable subdomains too.
The tag you have currently tracking pageviews on the main domain (example.com), provided it is using the All Pages trigger and is not restricted to only fire on the main domain, will then also track the subdomain pageviews as well.
Once the GTM container code is sorted out, ...
You need to add your Google Tag Manager code in your website.
Then, you have two options:
Leave it, don't do anything else. Your Google Analytics code will remain as it is and FB Pixel will be added via Google Tag Manager.
Alternatively, you can remove the code for Google Analytics from your website and add a tag in Google Tag Manager https://support....
you can leave the global site tag tracking code snippet on your site, just do not also configure tags for GA pageview tracking within GTM.
If it were me, I would remove the global site tag tracking code snippet and configure tags for GA tracking within the google tag manager container.
Bit of extra information:
Both Tag Manager and gtag.js perform ...
DataLayer variables aren't looked at for Custom Event triggering. You'll need a different syntax to get your event to fire.
The GTM custom event documentation has an example similar to what you are trying to do. To trigger a custom event and also capture open/sent, you'll need 2 things pushed into the dataLayer.
Your variable is correctly set up to capture ...
An easy way is to create an event tag with a link click trigger where the clicked url does not contain your hostname, but watch out for clicks that land in elements without href attributes like for example jumplinks!