Adding new cards to Trello board via Slack channel is ok.
But I do not receive when someone post new cards to Trello board.
It sounds like you've added the Trello Slack app just fine, and that's working as intended. That app does not provide any notifications automatically from Trello.
For that, you want to enable the Slack Power-Up on your board, which can be configured to notify you of whatever activity you're looking for: http://help.trello.com/article/1049-slack-app
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