This form has some built-protections to prevent against automated/robot submissions.
There are two checks:
1) the form needs to be submitted at least 3 seconds after it was shown.
What browser are you using?
Do you use any plugins that might affect this. e.g. something that auto-fills forms?
Do you see any errors in your browser's error console?
Can you post the URL of your site so I can see if there is anything obviously wrong?
You can ignore this. It means that the dev tools are trying to load debug info from the server - which will only exist on dev builds of webtrees. It doesn't affect anything. It just allows the browser to display "prettier" error messages.
> I was using Chrome but tried IE and it works
OK - my guess is that chrome (or a chrome plugin?) is doing something "clever".
Does it work if you use an "incognito mode" window in chrome?