here your code with a few changes:
function sendemail() {
var ss = SpreadsheetApp.getActiveSheet();
// var ss = SpreadsheetApp.getActiveSpreadsheet();
// var sheet = ss.getSheetByName("Visitors Log");
var user = Session.getActiveUser();
var row = ss.getActiveCell().getRow();
// var activecell = sheet.getActiveCell().getRow();
var vals = ss.getRange(row, 2, 1, 4).getValues();
var startdate = Utilities.formatDate(new Date(vals[0][0]), 'IST', 'dd/MMM/yyyy');
var enddate = Utilities.formatDate(new Date(vals[0][1]), 'IST', 'dd/MMM/yyyy');
var visitorsname = vals[0][2];
var designation = vals[0][3];
var body =""
var thStyle = "padding:5px;";
var tdStyle = "font-style:oblique;background-color:#BDBDBD";
var tableStyle = "border:1px"
body += "<table style='"+tableStyle+"'><tr>"+
"<th style='"+thStyle+"'>Start Date </th>"+
"<td style='"+tdStyle+"'>"+startdate + "</td></tr>"+
"<tr><th style='"+thStyle+"'>End Date </th>" +
"<td style='"+tdStyle+"'>"+enddate + "</td></tr>"+
"<tr><th style='"+thStyle+"'>Visitor's Name</th>"+
"<td style='"+tdStyle+"'>"+visitorsname +"</td></tr>"+
"<tr><th style='"+thStyle+"'>Designation</th>"+
"<td style='"+tdStyle+"'>"+designation+"</td></tr>"+
"</table>";
var cc="";
MailApp.sendEmail(user,"Visitor Details",body,{htmlBody: body, cc:cc});
}