Hello.
I have tried to add second message on the topic which I posted one message before.
But as I am going to send, I am getting message your message couldn't be send.

Hello,
this is because you are not allowed to post twice in a row on the same topic.

On the client you don't get the proper error, but on the website it will tell you so. You must edit your previous message if you have something to add. That is also possible from the website only.

Thanks for the info!