Failed to execute removeChild on Node

Other stack answers such as this and this seem to be specialized cases and I believe my case is more generalized. I am doing this in my js:

var markerDiv = document.createElement("div");
markerDiv.innerHTML = "<div id='MyCoolDiv' style='color: #2b0808'>123</div>";
document.getElementById("playerContainer").appendChild(markerDiv);

// after a brief delay, REMOVE the appended child
setTimeout(function(){ 
    var myCoolDiv = document.getElementById("MyCoolDiv");
    document.getElementById("playerContainer").removeChild(myCoolDiv);
}, 1500);

Everything works correctly and as expected (the div is correctly appended and I can see it) until removeChild() is called, at which time I get the error Failed to execute 'removeChild' on 'Node'.

What am I doing wrong?

Your myCoolDiv element isn't a child of the player container. It's a child of the div you created as a wrapper for it (markerDiv in the first part of the code). Which is why it fails, removeChild only removes children, not descendants.

You'd want to remove that wrapper div, or not add it at all.

Here's the "not adding it at all" option:

var markerDiv = document.createElement("div");
markerDiv.innerHTML = "<div id='MyCoolDiv' style='color: #2b0808'>123</div>";
document.getElementById("playerContainer").appendChild(markerDiv.firstChild);
// -------------------------------------------------------------^^^^^^^^^^^

setTimeout(function(){ 
    var myCoolDiv = document.getElementById("MyCoolDiv");
    document.getElementById("playerContainer").removeChild(myCoolDiv);
}, 1500);

<div id="playerContainer"></div>

Or without using the wrapper (although it's quite handy for parsing that HTML):

var myCoolDiv = document.createElement("div");
// Don't reall need this: myCoolDiv.id = "MyCoolDiv";
myCoolDiv.style.color = "#2b0808";
myCoolDiv.appendChild(
  document.createTextNode("123")
);
document.getElementById("playerContainer").appendChild(myCoolDiv);

setTimeout(function(){ 
    // No need for this, we already have it from the above:
    // var myCoolDiv = document.getElementById("MyCoolDiv");
    document.getElementById("playerContainer").removeChild(myCoolDiv);
}, 1500);

<div id="playerContainer"></div>