After installing the Zendesk integration, you can create tickets from any Slack channel. However, for notifications and Answer Bot to function in each channel, you need to invite the Zendesk app.
In Slack, go to the channel where you wish to invite the Zendesk app.
In the message bar, type `/invite @Zendesk` and then press the return key.
You can now set up notifications and Answer Bot settings for the channel. Please note, it may take about 10-15 minutes for a newly added channel to appear in the Slack for Zendesk Support app.