Javascript is object oriented.You use it to create an object such as a form or button on a web page ONSUBMIT is a function of the object You add code (to the code creating the form) to specify what happens when the button is pressed.
You get the error because the action the form is coded to take on pressing the button cannot be completed.The code might contain syntax errors or be designed to open another web page which also has some incorrect code on it.You can't do much about it .The website needs to correct the error.