VB.NET嵌套GridView出现Undefined问题求助

各位大神好！我最近在做VB.NET的嵌套GridView，要实现点击展开/收起子表格的功能，照着思路写了代码，结果碰上个棘手的问题——浏览器控制台一直报Undefined错误，点加号图标完全没反应，卡了好半天没头绪，求各位帮忙看看哪里出问题了！

先贴一下我目前的ASPX页面代码：

<asp:GridView ID="dgvPickups" runat="server" AutoGenerateColumns="false" CssClass="Grid" DataKeyNames="PickupID" OnRowDataBound="PickupRowDataBound" >
    <Columns>
        <asp:TemplateField>
            <ItemTemplate>
                <img alt="" style="cursor: pointer" src="Content/plus.png" />
                <img alt="" style="cursor: pointer; display: none" src="Content/minus.png" />
                <div style="display: none" class="childGrid">
                    <asp:GridView ID="dgvPickupDetails" runat="server" AutoGenerateColumns="false" CssClass="ChildGrid">
                        <Columns>
                            <asp:BoundField ItemStyle-Width="150px" DataField="ItemID" HeaderText="项目ID" />
                            <asp:BoundField ItemStyle-Width="150px" DataField="ItemName" HeaderText="项目名称" />
                        </Columns>
                    </asp:GridView>
                </div>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField ItemStyle-Width="150px" DataField="PickupID" HeaderText="取件ID" />
        <asp:BoundField ItemStyle-Width="150px" DataField="PickupDate" HeaderText="取件日期" />
    </Columns>
</asp:GridView>

我还写了控制展开收起的JS代码，逻辑就是点击加号切换成减号，同时显示子表格，反之隐藏：

$("img").click(function () {
    var row = $(this).closest("tr");
    var childDiv = row.find(".childGrid");
    var plusImg = row.find("img[src='Content/plus.png']");
    var minusImg = row.find("img[src='Content/minus.png']");
    
    childDiv.toggle();
    plusImg.toggle();
    minusImg.toggle();
});

后台VB.NET的RowDataBound事件里，我也写了绑定子GridView数据的逻辑：

Protected Sub PickupRowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
    If e.Row.RowType = DataControlRowType.DataRow Then
        Dim pickupID As Integer = Convert.ToInt32(dgvPickups.DataKeys(e.Row.RowIndex).Value)
        Dim childGrid As GridView = DirectCast(e.Row.FindControl("dgvPickupDetails"), GridView)
        childGrid.DataSource = GetPickupDetails(pickupID)
        childGrid.DataBind()
    End If
End Sub

Private Function GetPickupDetails(ByVal pickupID As Integer) As DataTable
    Dim dt As New DataTable()
    ' 这里是从数据库查询对应取件ID详情的逻辑，测试过能拿到正确数据
    Return dt
End Function

现在的问题是，页面加载完点击加号完全没反应，打开控制台一看，报的是Uncaught ReferenceError: $ is not defined。我明明已经在页面头部引用了jQuery了啊，路径也核对过没错：

<script src="Scripts/jquery-3.6.0.min.js"></script>

我自己排查了好半天，后台绑定数据是正常的，图片路径也没写错，就是前端点击没反应还报Undefined，有没有大神遇过类似的问题？麻烦给我指个方向！

备注：内容来源于stack exchange，提问作者Jason Guernsey