Without min api version -
const message = new RichMediaMessage(SAMPLE_RICH_MEDIA);
With min api version -
const message = new RichMediaMessage(SAMPLE_RICH_MEDIA, null, [], undefined, undefined, null, 7);
Both doesn't work while sending
response.send(message);
Normal text message works well
const message = new TextMessage('hi');
response.send(message);
I have found the issue.
In their api documentation,
https://developers.viber.com/docs/api/nodejs-bot-api/#RichMediaMessage
"ButtonsGroupRows": 2 (this should be => 4+1=5)
Changing "2" to "5" solved it.