Operation not allowed on a unidirectional dataset TSQLQuery

procedure TFMRelFuncPer.BitBtn2Click(Sender: TObject);
var
 data1, data2: string;
begin
DataModule1.SQLRelFuncPer.Close;
DataModule1.SQLRelFuncPer.sql.Clear;
DataModule1.SQLRelFuncPer.sql.add(´select * from tb_afastamento´);
DataModule1.SQLRelFuncPer.sql.add(´where afast_cod > 0´);
if CheckBox1.Checked=true then
 DataModule1.SQLRelFuncPer.sql.Add(´and afast_cod_servidor = ´ + DataModule1.SDSAfastamentoafast_cod_servidor.AsString);
if CheckBox2.Checked=true then
 begin
  Data1:=FormatDateTime(´YYYY/MM/DD´, StrtoDate(MaskEdit1.Text));
  Data2:=FormatDateTime(´YYYY/MM/DD´, StrtoDate(MaskEdit2.text));
  DataModule1.SQLRelFuncPer.sql.add(´and afast_data_inic >= ´ + data1);
  DataModule1.SQLRelFuncPer.SQL.Add(´and afast_data_fim =< ´ + data2);
 end;
DataModule1.SQLRelFuncPer.open;
if DataModule1.SQLRelFuncPer.RecordCount = 0 then
 Application.MessageBox(´Não foi selecionado nenhum registro´,´Atenção´, MB_OK + MB_ICONEXCLAMATION)
 else
  RvProject1.Execute;
end;