Try
            Dim strCon As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data" & _
              "Source=cao.mdb"
 
            Dim con As New OleDb.OleDbConnection
 
            Dim DS As New DataSet
 
            con.ConnectionString = strCon
 
            Dim sqladapter As OleDb.OleDbDataAdapter
            Dim sqlstring As String
 
            sqlstring = "SELECT * FROM JOURNAL WHERE VRENUM = '" & rnr & "'"
            MsgBox(sqlstring)
            sqladapter = New OleDb.OleDbDataAdapter(sqlstring, con)
 
            con.Open()
 
            sqladapter.Fill(DS)
 
            MsgBox(sqladapter.GetFillParameters)
 
            MessageBox.Show("Verbindung erfolgreich hergestellt")
 
            con.Close()
        Catch ex As Exception
            MsgBox("Fehler: " & ex.Message)
        End Try
    End Function

Das Argument  Prompt kann nicht in den Typ String umgewandelt werden.