error conversion failed when converting the varchar value 'INT' to data type int -how to solve

Public Class PurchaseOrder
    Inherits System.Web.UI.Page
    Dim strcon As String = ConfigurationManager.ConnectionStrings("DeviceDBConnectionString").ToString
    Dim con As SqlConnection = New SqlConnection(strcon)
    Dim cmd As New SqlCommand
    Dim dr As SqlDataReader

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        BindGridView()
        GetCompanyDetails()
        GetDepartmentDetails()
    End Sub

    Protected Sub Button5_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button5.Click
        Try
            con.Open()
            cmd.Connection = con
            cmd.CommandText = "INSERT INTO TBL_PURCHASE(PONO, POHDATE,COMPCODE,POAMOUNT,POWARRANTY,POEHDATE,DCCODE)VALUES('" & txtpn.Text & "','" & txtpurdate.Text & "','" & ddlcmp.SelectedValue & "','" & txtamt.Text & "','" & txtwarperiod.Text & "','" & txtdoe.Text & "','" & ddldept.SelectedValue & "')"
            'cmd.CommandText = "INSERT INTO TBL_PURCHASE(PONO, POHDATE,COMPCODE,POAMOUNT,POWARRANTY,POEHDATE,DCCODE)" &_VALUES(@PONO, @POHDATE,@COMPCODE,@POAMOUNT,@POWARRANTY,@POEHDATE,@DCCODE)"
           
            cmd.ExecuteNonQuery()
            '
        Catch ex As Exception
            'MsgBox.Show("Error while inserting record on table..." & ex.Message, "Insert Records")
            MsgBox("Error " & ex.Message)
        Finally
            con.Close()
        End Try
    End Sub

    Protected Sub GridView1_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles GridView1.SelectedIndexChanged
        txtpn.Text = GridView1.SelectedRow.Cells(1).Text
        txtpurdate.Text = GridView1.SelectedRow.Cells(2).Text
        If GridView1.SelectedRow.Cells(3).Text = " " Then
            txtamt.Text = ""
        Else
            txtamt.Text = GridView1.SelectedRow.Cells(3).Text
        End If
        txtwarperiod.Text = GridView1.SelectedRow.Cells(4).Text
        txtdoe.Text = GridView1.SelectedRow.Cells(5).Text
        If GridView1.SelectedRow.Cells(6).Text Is Nothing Then

            'ddlcmp.SelectedValue = GridView1.SelectedRow.Cells(6).Text
        Else
            ddlcmp.SelectedValue = GridView1.SelectedRow.Cells(6).Text
        End If
        If GridView1.SelectedRow.Cells(7).Text Is Nothing Then
        Else
            ddldept.SelectedValue = GridView1.SelectedRow.Cells(7).Text
        End If



        

    End Sub

    

    Private Function SqlConnection(ByVal p1 As String) As SqlConnection
        Throw New NotImplementedException
    End Function
    Protected Sub GetCompanyDDL()
        Try
            cmd.Connection = con
            cmd.CommandText = "SELECT [COMPCODE], [COMPNAME] FROM [TBL_COMPANY_COD]"
            cmd.Connection.Open()
            dr = cmd.ExecuteReader()
            'ddlSignatory.Items.Add("")
            While dr.Read()
                Dim newListItem As New ListItem()
                newListItem.Text = dr.GetString(1).ToString()
                newListItem.Value = dr.GetValue(0).ToString()
                ddlcmp.Items.Add(newListItem)
            End While
            cmd.Connection.Close()
        Catch ex As Exception
            ddlcmp.Items.Add("")
            cmd.Connection.Close()
        End Try
    End Sub
    Protected Sub GetCompanyDetails()

        Dim da As New SqlDataAdapter("SELECT [COMPCODE], [COMPNAME] FROM [TBL_COMPANY_COD]", strcon)
        Dim dt As New Data.DataTable
        da.Fill(dt)
        da.Dispose()
        ddlcmp.DataSource = dt
        ddlcmp.DataTextField = "COMPNAME"
        ddlcmp.DataValueField = "COMPCODE"
        ddlcmp.DataBind()
        'ddlcmp.ClearSelection()
    End Sub
    Protected Sub GetDepartmentDetails()

        Dim da As New SqlDataAdapter("SELECT [DEPCODE], [DEPDESC] FROM [TBL_DEPARTMENT_COD]", strcon)
        Dim dt As New Data.DataTable
        da.Fill(dt)
        da.Dispose()
        ddldept.DataSource = dt
        ddldept.DataTextField = "DEPCODE"
        ddldept.DataValueField = "DEPDESC"
        ddldept.DataBind()
        'ddlcmp.ClearSelection()
    End Sub
    Protected Sub GetDepartmentDDL()
        Try
            cmd.Connection = con
            cmd.CommandText = "SELECT [DEPCODE], [DEPDESC] FROM [TBL_DEPARTMENT_COD]"
            cmd.Connection.Open()
            dr = cmd.ExecuteReader()
            'ddlSignatory.Items.Add("")
            While dr.Read()
                Dim newListItem As New ListItem()
                newListItem.Text = dr.GetString(1).ToString()
                newListItem.Value = dr.GetValue(0).ToString()
                ddldept.Items.Add(newListItem)
            End While
            cmd.Connection.Close()
        Catch ex As Exception
            ddldept.Items.Add("")
            cmd.Connection.Close()
        End Try
    End Sub

    Protected Sub BindGridView()
        Dim da As New SqlDataAdapter("SELECT [PONO], [POHDATE], [POAMOUNT], [POWARRANTY], [POEHDATE], [COMPCODE], [DCCODE] FROM [TBL_PURCHASE]", strcon)
        Dim ds As New DataSet()
        da.Fill(ds)
        GridView1.DataSource = ds
        GridView1.DataBind()
    End Sub


    Protected Sub Button4_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnupdate.Click
        Try
            con.Open()
            cmd.Connection = con
            cmd.CommandText = "UPDATE TBL_PURCHASE SET PONO ='" & txtpn.Text & "', POHDATE = '" & txtpurdate.Text & "',COMPCODE = '" & ddlcmp.SelectedValue & "',POAMOUNT= '" & txtamt.Text & "',POWARRANTY = '" & txtwarperiod.Text & "',POEHDATE= '" & txtdoe.Text & "',DCCODE= '" & ddldept.SelectedValue & "'WHERE PONO= '" & txtpn.Text & "'"

            cmd.ExecuteNonQuery()

        Catch ex As Exception
            'MsgBox.Show("Error while inserting record on table..." & ex.Message, "Insert Records")
            MsgBox("Error " & ex.Message)
        Finally
            con.Close()
        End Try
    End Sub
    Public Sub ClearTextBox(ByVal root As Control)
        For Each ctrl As Control In root.Controls
            ClearTextBox(ctrl)
            If TypeOf ctrl Is TextBox Then
                CType(ctrl, TextBox).Text = String.Empty
            End If
        Next ctrl
    End Sub
    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
        ClearTextBox(Me)
    End Sub
End Class