Pivot or...?

okay, i think i've fixed that bug. Try this: 

let
    Source = your_table,
    f = (tbl as table) as table => 
        [rows = Table.ToRecords(tbl),
        acc = 
            List.Accumulate(
                rows,
                {},
                (s, c) => 
                    if c[CLOCK_TYPE] = 51 then s & {c & [CHECKIN = c[DATETIMECHECK]] & [CHECKOUT = null]} 
                    else if List.IsEmpty(s) 
                        then s & {c & [CHECKIN = null] & [CHECKOUT = c[DATETIMECHECK]]}
                        else if List.Last(s)[CHECKOUT] = null 
                            then List.RemoveLastN(s, 1) & {Record.TransformFields(List.Last(s), {"CHECKOUT", each c[DATETIMECHECK]})}
                            else s & { c & [CHECKIN = null] & [CHECKOUT = c[DATETIMECHECK]]}     
            ),
        out = Table.FromRecords(acc)][out],
    g = Table.Group(Source, {"SHIFT_NUMBER", "EMPLOYEE_CODE"}, {"all", each f(Table.Sort(_, "ABS_NUMBER"))}),
    expand = Table.ExpandTableColumn(g, "all", {"CLOCK_DATE", "CHECKIN", "CHECKOUT"})
in
    expand