How to debug form submit ajaxsubmit(ajaxOptions)?

Question

See below code, $('#form').valid() is true but data === 'validation_error'.

Anyone could tell me how to debug to find out why I got the validation error even form is valid?

App.SubmitApplication = () => {
    $('#form').submit((e) => {
        e.preventDefault();
        e.stopImmediatePropagation();
        
        if ($('#form').valid()) {
            let ajaxOptions = {
                beforeSend: () => {
                    helperApp.BlockUI('js-pnl-application');
                },
                success: (data) => {
                    if (data === 'validation_error') {
                        //show validation error
                    }
                    //show SUCCESS
                 },
                 error: (xhr, textStatus, errorThrown) => {
                    //An error happened when saving your data.");
                 }
             };
             
             $('#form').ajaxSubmit(ajaxOptions);
        }
    })
};