Can't findcontrol linkbutton on nested child gridview

You're trying to find the LinkButton within the parent GridView (gvDepartment) instead of the child GridView (gvEmployees). Another thing the lnkbtn object reference error occurs because lnkbtn is being searched for in the parent row, where it doesn't exist as expected.

Protected Sub gvDepartment_RowDataBound(sender As Object, e As GridViewRowEventArgs) Handles gvDepartment.RowDataBound
    If e.Row.RowType = DataControlRowType.DataRow Then
        ' Get the primary key value for the current row
        Dim id As String = gvDepartment.DataKeys(e.Row.RowIndex).Value.ToString()

        ' Find the child GridView (gvEmployees) inside the current row
        Dim gvChild As GridView = TryCast(e.Row.FindControl("gvEmployees"), GridView)

        If gvChild IsNot Nothing Then
            ' Bind the child GridView with the appropriate data
            gvChild.DataSource = GetData2(String.Format("SELECT * FROM tblFiles WHERE ID='{0}'", id))
            gvChild.DataBind()

            ' Loop through each row in the child GridView
            For Each childRow As GridViewRow In gvChild.Rows
                If childRow.RowType = DataControlRowType.DataRow Then
                    ' Find the LinkButton inside the child GridView
                    Dim lnkbtn As LinkButton = TryCast(childRow.FindControl("lnkDownload"), LinkButton)

                    ' Check if the first cell is empty and hide the LinkButton if needed
                    If childRow.Cells(0).Text.Trim() = "" Then
                        If lnkbtn IsNot Nothing Then
                            lnkbtn.Visible = False
                        End If
                    End If
                End If
            Next
        End If
    End If
End Sub