window.onload is not a function

This error message usually appears when you have defined a function called "window.onload" but it is not actually a function.

To fix this error, you need to make sure that the "window.onload" function has been defined properly. Here are a few things you can try:

1. Check that the function name is spelled correctly and that there are no typos.
2. Make sure that the function is defined before it is called.
3. Check that there are no conflicting scripts or libraries that could be causing issues.
4. Try wrapping your code in a "DOMContentLoaded" event listener instead of "window.onload".

Here's an example of how to define a "DOMContentLoaded" event listener:

document.addEventListener("DOMContentLoaded", function(event) {
  // Your code here
});

This will ensure that your JavaScript code is executed once the page has finished loading.