The check

The check asks your measurement server itself what it can serve, so you do not have to open Tag Manager to find out whether the pieces are in place. It loads separately from the page: a measurement server that has stopped answering should not hold up the list.

What it tests

Does the measurement server answer? If not, that is all the check says — the rest falls away, because without an answer there is nothing further to ask.

Does it serve the snippet? Only if your web container ID is filled in. Tagyard asks for the snippet for that ID; if it comes back, the chain from your site to your measurement server stands.

If the field is empty, the check says the loader cannot be checked. That is not an error: your measurement server works fine, only this test cannot run.

When it does not serve the snippet

Then there are three possible causes, and the message names all three:

1. No version has been published in your server container yet.
2. The client Google Tag Manager: Web Container is missing.
3. This ID is not listed under Allowed Container IDs.

From the outside it is impossible to see which of the three it is. All three return the same answer from your measurement server, and naming a guess would send you to the wrong setting. Walk through them in Tag Manager in this order; the first is the most common cause.

What the check is not

The check looks at whether your measurement server can serve the snippet. It says nothing about whether measurements are arriving, and nothing about whether those measurements reach Google or Meta. There are two other places for that: the verdict for the state of affairs in plain language, and the traffic log for the individual requests.