"int" is a struct and cannot be null. Nullable<int> is a class that can be null. The common shortcut is int? which works almost the same as int.
Some differences are that you need to use the Value property to get the non-null value. It returns the default<int> when it's null which is zero (0).
DataTable PreviousQuarter = GroupFive.Tables["PreviousQuarter_108_49_100_51"];
LastQuarter.Merge(PreviousQuarter);
var AHCT = (from ah in PreviousQuarter.AsEnumerable()
where
ah.Field<int?>("ParameterID").Value == 10 || ah.Field<int?>("ParameterID") == 11
|| ah.Field<int?>("ParameterID").Value == 12 || ah.Field<int?>("ParameterID") == 13
select ah.Field<int?>("Tot_Tkts_Closed")).ToList();
if (AHCT == null)
{
Response = "";
}