As you are not actually submitting a form (to a server) you should use <input type = "button" instead of "submit".
document.write() is obsolete - use DOM methods such as innerHTML to display the result of the calculation in a <span>
document.getElementById("result").innerHTML = payment.toFixed(2); // display 2 decimal places
It would also be a good idea to verify that the values entered by the users are numbers.
Your variable principal
is defined by has no value (you call it amount
Assigning a name to a form is now deprecated - prefer to use id instead. You can still refer to the form by document.forms if you wish.
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