Answers for "app.min.js:1 Uncaught (in promise) TypeError: Illegal invocation"

TypeError: Illegal invocation

Your datatype is not JSON, it’s a FormData. And for jQuery to send a FormData, it needs…
$.ajax({
    type: "POST",
    url: 'signin2.php',
    data: formData,
    processData: false, //add this
    contentType: false, //and this
})

jquery-3.4.1.min.js:2 Uncaught (in promise) TypeError: Illegal invocation

var form = $('#AddPersonnelForm')[0];
var frmData = new FormData(form);



processData: false,
contentType: false,