You are creating a text node, not an element. You need to create an element and give it an id to be able to use getElementById
.
I don't know of any reasonable way to search for a text node, although you could always check the text nodes of the element you attached it to and see if it's there.
var message = "ERROR";
var t = document.createTextNode(message);
var node = document.getElementById('content').appendChild(t);
if (document.getElementById('content').innerHTML !== message) {
console.log('element not added');
} else {
console.log('element added');
}
Here is a fiddle: