You are treating the JSP code as if its a Javascript string variable. That's going to be a syntax error in your browser. You should just do:
var editModify = "<td><%=Site.getFieldUIName(Site.field.site_name.name())%></td>";
Or, if you want, wrap it in quotes to make it end up as a Javascript string literal:
var editModify = "<td>" + "<%=Site.getFieldUIName(Site.field.site_name.name())%>" + "</td>";