Please check the following in the Slack app:


1. Make sure "Allow Slash Commands" is checked in the app configuration page. Please navigate to Admin -> Support Operations -> Apps -> click on slash integration to see the settings (gear icon) where you could see this configuration. 

2. Make sure that the correct Slash command token (obtainable when you create the slash command) is copied to Freshdesk-Slack app settings page.

3. Make sure the <user API token> entered along with the /fd_ticket command is the right one.