Getting null when trying to send Email via Javascript

You might be receiving emails but in Junk Mail folder. So You have to pass all the attributes e.g Body, Subject, To, From as a variable rather than static.

Please check the updated code below :

function sendEmail() {

var appweburl = _spPageContextInfo.webAbsoluteUrl;
var urlTemplate = appweburl + "/_api/SP.Utilities.Utility.SendEmail";


var fieldName = "Notificar a";
var _PeoplePicker = $("div[title='" + fieldName + "']");
var emailSpan = $(_PeoplePicker).find('.sp-peoplepicker-userSpan');
var _sid ="";
if (emailSpan.length > 0) {
         _sid = _sid = $(emailSpan).attr('sid');
     _sid =  _sid.split("|")[2];
} else {
       alert("null");
}

        var fieldUsuario = _sid;
        var fromMail = '[email protected]';
        var subject = 'remember ';
        var body = 'Hello';    

    $.ajax({
        contentType: 'application/json',
        url: urlTemplate,
        type: "POST",
        data: JSON.stringify({
            'properties': {
                '__metadata': { 'type': 'SP.Utilities.EmailProperties' },
                'From': fromMail,
                'To': { 'results': [fieldUsuario] },
                'Body': body, 
                'Subject': subject
            }
        }
      ),
        headers: {
            "Accept": "application/json;odata=verbose",
            "content-type": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },
        success: function (data) {
            var result = data.d.results;
            var i = result.length;
        },
        error: function (err) {
            alert(JSON.stringify(err));
        }
    });
}