Update code as below:
function checkAccountingStatus() {
var accountingstatus = Xrm.Page.getAttribute('accountratingcode');
if(accountingstatus!=null)
{
var accountingstatusValue = Xrm.Page.getAttribute('accountratingcode').getSelectedOption();
if (accountingstatus.Valuevalue == 282740001) {
Xrm.Utility.alertDialog("The Account is in Collections.");
}
}
}