Dim dr As DataRow = e.DataRow
Dim pr As DataRow
Dim tr As DataRow
If dr("客户名称") IsNot Nothing AndAlso dr("收款日期") IsNot Nothing Then
    Dim filter As String =" 客户名称= \'" & dr("客户名称") & "\'"
    pr = DataTables("客户一览表").Find(filter)
    tr = DataTables("收款明细表").Find(filter)
    If pr IsNot Nothing Then
        Dim val As Integer
        Dim cmd As New SQLCommand
        cmd.CommandText = "Select Sum(收款总额) Fr om {收款明细表} Where year(收款日期) = \'" & Date.today.year & "\'"
        val  = cmd.ExecuteScalar()
        pr("本期发生_已收货款")=val
        MessageBox.show(pr("本期发生_已收货款"))
    End If
End If

提示信息：至少一个参数未指定。

列名肯定写错了，或者表格里没有此列，表达式列不能直接写。

cmd.CommandText = "Select Sum(收款总额) Fr om {收款明细表} Where year(收款日期) = \'" & Date.today.year & "\'"