I'm not sure, but you set id:ÏD but it is not in the colModel.

Irecommend you to set key: true in a column which has unique values by example:

name: 'TeamID', index: 'TeamID', width: 55, search: true, sorttype: 'int', key: true

Also can you please post a simple data returned from the server?, so that I can see what is happen.
Thank you

Kind Regards

I have already checked other similar questions and their answers, but none of them helped me solve my issue. Maybe someone can check my jqGrid script below and tell me what is wrong with it or what is there missing. For the reference, I am using the version "4.5.2" of jqGrid.

$('#tblTargetDetails').jqGrid({
    url: 'PostHandlers/CommonHandler.asmx/GetTargetDetails',
    datatype: 'json',
    ajaxGridOptions: { contentType: 'application/json; charset=utf-8' },
    ajaxRowOptions: { contentType: "application/json; charset=utf-8", dataType: "json" },
    postData: "{'TargetID': '" + TargetID + "'}",
    mtype: 'POST',
    colNames: ['Team ID', 'Member UserID', 'Measure', 'Product', 'Month1', 'Month2'],
    colModel: [
      { name: 'TeamID', index: 'TeamID', width: 55, search: true, sorttype: 'int' },
      { name: 'MemberUserID', index: 'MemberUserID', width: 90, search: true, stype: 'text' },
      { name: 'Measure', index: 'Measure', width: 90, search: true, searchoptions: {} },
      { name: 'Product', index: 'Product', width: 90, search: true },
      { name: 'Month1', index: 'Month1', width: 80, editable: true, search: true },
      { name: 'Month2', index: 'Month2', width: 80, editable: true, search: false }],
    jsonReader: {
        root: "d.TargetDetails",
        records: "d.RecordsCount",
        id: "ID",
        cell: "",
        page: function () { return 1; },
        total: function () { return 1; },
    },
    pager: '#pnlTargetDetails',
    rowNum: 50,
    rowTotal: 2000,
    rowList: [20, 30, 50],
    loadonce: true,
    viewrecords: true,
    gridview: true,
    ignoreCase: true,
    rownumbers: true,
    caption: 'Target Details',
    editurl: 'PostHandlers/CommonHandler.asmx/EditTargetDetail',
    serializeRowData: function (postData) {

        return JSON.stringify({ 'content': JSON.stringify(postData), 'UserID': UserID });
    },
    onSelectRow: function (id) {
        if (id && id !== lastSel) {
            $('#tblTargetDetails').restoreRow(lastSel);
            lastSel = id;
        }
        $('#tblTargetDetails').jqGrid('editRow', id, true);

    }
});

$('#tblTargetDetails').jqGrid('navGrid', '#pnlTargetDetails', { search: true, edit: false, add: false, del: false });

$('#tblTargetDetails').jqGrid('filterToolbar', { stringResult: true, searchOnEnter: false });