C# – When reading a Cursor from a resources file an ArgumentException is thrown

Please try using this.serialPort1.RtsEnable = true

Suggested based on the stack trace of your exception

at System.IO.Ports.SerialStream.set_RtsEnable(Boolean value)
at System.IO.Ports.SerialStream..ctor(String portName, Int32 baudRate, Parity parity, Int32 dataBits, StopBits stopBits, Int32 readTimeout, Int32 writeTimeout, Handshake handshake, Boolean dtrEnable, Boolean rtsEnable, Boolean discardNull, Byte parityReplace)
at System.IO.Ports.SerialPort.Open()

Just calling ExecuteReader is not enough. Initially any kind of DataReader is positioned before any row retrieved. You need to call Read on the reader to position it on the first row returned

SQLiteDataReader data = db.returnDataReader(.....);
if(data.Read())
{
    string res = data["created_at"].ToString();
    ....
}

or add a loop

SQLiteDataReader data = db.returnDataReader(.....);
while(data.Read())
{
     ..get data from your rows here
}

However I don't consider your approach to return a SqlLiteDataReader a good practice. The reader depends on the connection being open and the connections are objects that you should really close as fast as possible to avoid dangerous resources leaks.

I suggest to use another approach to read you data

public void ReadData(string txtQuery, Action<SQLiteDataReader> loader)
{
    using(SQLiteConnection con = Openconn())
    using(SQLiteCommand cmd = new SQLiteCommand(txtQuery, con))
    using(SQLiteDataReader rd = cmd.ExecuteReader())
    {
        while(rd.Read())
            loader(rd);
    }
}

Then you call this method passing the delegate to the method that reads your data

ReadData(...querytext..., readCreatedData);

and then write the method readCreatedData that will be called by the code where you open the connection and the other disposable objects.

void readCreatedData(SQLiteDataReader data)
{
     string res = data["created_at"].ToString();
     .....
}

Finally as Rufo pointed in its comment you are really just reading a single value from a single column and this is better done using a simple ExecuteScalar

public string ReadString(string txtQuery)
{
    using(SQLiteConnection con = Openconn())
    using(SQLiteCommand cmd = new SQLiteCommand(txtQuery, con))
    {
        object result = cmd.ExecuteScalar();
        return (result == null ? "" : result.ToString());
    }
}