It seems you doing this error due to case sensitivity please use the following wvode to create an formdata object . you can also send data to backend using form data.

let formData = new FormData(document.getElementByClassName('someClass'));

You can build a FormData object yourself, instantiating it then appending fields to it by calling its append() method, like this

const formData = new FormData();

formData.append("email", "email@gmail.com");
formData.append("email2", "email@gmail.com");