How to filter the customer's first sales invoice in Cust.Ledger Entry

Just some tips:

• GETRANGEMIN and GETRANGEMAX gets the minimum and maximum values of a filter range. In your sample, you don't have a value in your date filter, so that's why the system is giving you an error
• To get the minimum date and the maximum date from a set of records, you need to first sort the records by the date, then do a FINDFIRST to read the first record, and then do a FINDLAST to read the last record. You can then use the date field value to set the filters. I don't understand why you would set that filter though, because with those values you will get all records anyway, so that filter is not effective

I didn't run this report, but this shows where to put the code and how to use some of the AL commands that you are trying to use

report 50100 MyReport
{
    Caption = 'My Sample CLE Report';
    UsageCategory = Lists;
    ApplicationArea = All;

    dataset
    {
        dataitem("Cust. Ledger Entry"; "Cust. Ledger Entry")
        {
            // use the OnPreDataItem trigger because this runs before the
            // records are retrieved from the database
            // The OnAfterGetRecord trigger is too late
            trigger OnPreDataItem()
            var
                MyCLE: Record "Cust. Ledger Entry";
                StartDate: Date;
                EndDate: Date;
                DateRangeMsg: Label 'Start date is %1 and the end date is %2';
            begin
                // using a separate variable to leave the dataitem alone
                MyCLE.Reset();
                // sorting the table to get them in the right order
                MyCLE.SetCurrentKey("Posting Date");

                // get the first record for the lowest data
                if MyCLE.FindFirst() then
                    StartDate := MyCLE."Posting Date"
                else
                    StartDate := 0D;

                // get the last record for the highest date
                if MyCLE.FindLast() then
                    EndDate := MyCLE."Posting Date"
                else
                    EndDate := 99991231D;

                // Now set the date range filter for the dataitem
                "Cust. Ledger Entry".SetRange("Posting Date", StartDate, EndDate);

                // you could also use SetFilter
                "Cust. Ledger Entry".SetFilter("Posting Date", '%1..%2', StartDate, EndDate);

                // now you could do GetRangeMin and GetRangeMax
                Message(DateRangeMsg, "Cust. Ledger Entry".GetRangeMin("Posting Date"),
                                        "Cust. Ledger Entry".GetRangeMax("Posting Date"));
            end;
        }
    }
}

You should really have a senior that can answer these types of questions. If you don't have a senior, then I would suggest that you get the programming book by David Studebaker.