Cannot read property 'getElementById' of undefined

This error message typically occurs when the JavaScript code is trying to access an element on the HTML document that does not exist. The error message indicates that the variable or object that is supposed to represent the HTML document is undefined, which means that the code cannot access the desired element.

To fix this error, you can check the following:

1. Make sure that the HTML document is properly loaded before the JavaScript code is executed. You can use the window.onload event to ensure that the page has fully loaded.

2. Check that the ID of the element you are trying to access is correct and matches the ID in the HTML code.

3. Verify that the JavaScript code is properly placed and is correctly linked to the HTML document.

4. If you are using a framework or library such as jQuery, make sure that it has been properly included and initialized.

5. Ensure that the script tag containing the JavaScript code is placed after the HTML elements it is trying to access.

By addressing the above issues, you can fix the error and access the desired element on the HTML document.