Refer below code and replace the btn_add_Click event code with the below.

Gridview textbox get enable true when adding rows

C#

protected void btn_add_Click(object sender, EventArgs e)
{
    int rowIndex = 0;
    if (ViewState["CurrentTable"] != null)
    {
        DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
        DataRow drCurrentRow = null;
        if (dtCurrentTable.Rows.Count > 0)
        {
            for (int i = 1; i <= dtCurrentTable.Rows.Count; i++)
            {
                //extract the TextBox values
                TextBox box1 = (TextBox)gvtrans.Rows[rowIndex].Cells[1].FindControl("txtcode");
                DropDownList box2 = (DropDownList)gvtrans.Rows[rowIndex].Cells[2].FindControl("ddlfourdr");
                TextBox box3 = (TextBox)gvtrans.Rows[rowIndex].Cells[3].FindControl("txtDebit");
                TextBox box4 = (TextBox)gvtrans.Rows[rowIndex].Cells[4].FindControl("txtCredit");
                DropDownList box5 = (DropDownList)gvtrans.Rows[rowIndex].Cells[5].FindControl("ddlType");
                if (box3.Text != "" || box4.Text != "")
                {
                    drCurrentRow = dtCurrentTable.NewRow();
                    drCurrentRow["RowNumber"] = i + 1;
                    dtCurrentTable.Rows[i - 1]["txtcode"] = box1.Text;
                    dtCurrentTable.Rows[i - 1]["ddlfourdr"] = box2.Text;
                    dtCurrentTable.Rows[i - 1]["txtDebit"] = box3.Text;
                    dtCurrentTable.Rows[i - 1]["txtCredit"] = box4.Text;
                    dtCurrentTable.Rows[i - 1]["ddlType"] = box5.Text;
                    rowIndex++;
                }
            }
            if (drCurrentRow != null)
            {
                dtCurrentTable.Rows.Add(drCurrentRow);
            }
            ViewState["CurrentTable"] = dtCurrentTable;
            gvtrans.DataSource = dtCurrentTable;
            gvtrans.DataBind();

            foreach (GridViewRow row in gvtrans.Rows)
            {
                TextBox txtDebit = (TextBox)row.FindControl("txtDebit");
                TextBox txtCredit = (TextBox)row.FindControl("txtCredit");
                DataTable dt = (DataTable)ViewState["CurrentTable"];
                string type = dt.Rows[row.RowIndex]["ddlType"].ToString();
                if (!string.IsNullOrEmpty(dt.Rows[row.RowIndex]["txtDebit"].ToString())
                    || !string.IsNullOrEmpty(dt.Rows[row.RowIndex]["txtCredit"].ToString()))
                {
                    txtDebit.Enabled = false;
                    txtCredit.Enabled = false;
                    if (type == "1")
                    {
                        txtCredit.Enabled = true;
                    }
                    if (type == "2")
                    {
                        txtDebit.Enabled = true;
                    }
                }
            }
        }
    }
    else
    {
        Response.Write("ViewState is null");
    }
    //Set Previous Data on Postbacks
    SetPreviousData();
}

VB.Net

Protected Sub btn_add_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim rowIndex As Integer = 0
    If ViewState("CurrentTable") IsNot Nothing Then
        Dim dtCurrentTable As DataTable = CType(ViewState("CurrentTable"), DataTable)
        Dim drCurrentRow As DataRow = Nothing
        If dtCurrentTable.Rows.Count > 0 Then
            For i As Integer = 1 To dtCurrentTable.Rows.Count
                Dim box1 As TextBox = CType(gvtrans.Rows(rowIndex).Cells(1).FindControl("txtcode"), TextBox)
                Dim box2 As DropDownList = CType(gvtrans.Rows(rowIndex).Cells(2).FindControl("ddlfourdr"), DropDownList)
                Dim box3 As TextBox = CType(gvtrans.Rows(rowIndex).Cells(3).FindControl("txtDebit"), TextBox)
                Dim box4 As TextBox = CType(gvtrans.Rows(rowIndex).Cells(4).FindControl("txtCredit"), TextBox)
                Dim box5 As DropDownList = CType(gvtrans.Rows(rowIndex).Cells(5).FindControl("ddlType"), DropDownList)
                If box3.Text <> "" OrElse box4.Text <> "" Then
                    drCurrentRow = dtCurrentTable.NewRow()
                    drCurrentRow("RowNumber") = i + 1
                    dtCurrentTable.Rows(i - 1)("txtcode") = box1.Text
                    dtCurrentTable.Rows(i - 1)("ddlfourdr") = box2.Text
                    dtCurrentTable.Rows(i - 1)("txtDebit") = box3.Text
                    dtCurrentTable.Rows(i - 1)("txtCredit") = box4.Text
                    dtCurrentTable.Rows(i - 1)("ddlType") = box5.Text
                    rowIndex += 1
                End If
            Next

            If drCurrentRow IsNot Nothing Then
                dtCurrentTable.Rows.Add(drCurrentRow)
            End If

            ViewState("CurrentTable") = dtCurrentTable
            gvtrans.DataSource = dtCurrentTable
            gvtrans.DataBind()

            For Each row As GridViewRow In gvtrans.Rows
                Dim txtDebit As TextBox = CType(row.FindControl("txtDebit"), TextBox)
                Dim txtCredit As TextBox = CType(row.FindControl("txtCredit"), TextBox)
                Dim dt As DataTable = CType(ViewState("CurrentTable"), DataTable)
                Dim type As String = dt.Rows(row.RowIndex)("ddlType").ToString()
                If Not String.IsNullOrEmpty(dt.Rows(row.RowIndex)("txtDebit").ToString()) _
                    OrElse Not String.IsNullOrEmpty(dt.Rows(row.RowIndex)("txtCredit").ToString()) Then
                    txtDebit.Enabled = False
                    txtCredit.Enabled = False
                    If type = "1" Then
                        txtCredit.Enabled = True
                    End If

                    If type = "2" Then
                        txtDebit.Enabled = True
                    End If
                End If
            Next
        End If
    Else
        Response.Write("ViewState is null")
    End If

    SetPreviousData()
End Sub