I have a Slack service that has been running for years. We have dozens of channels both public and private.
We recently started using Zendesk for customer support ticket management. I have enabled the Slack/Zendesk integraiton app. I am an Admin of both Zendesk and Slack.
When I run the install helper to specify which channel to send ticket notifications to - it only show's me 2 channels in my Slack workspace. I have dozens. I have added Zendesk to a few other channels, and yet, they do not show up as potential targets to send Zendesk messages to. I can't directly type a channel name in the input form - only the 2 channels offered are allowed to be set.
Showing the 2 channels that I can set ... :
Showing a subset of some of the channels (private) that are available. NOTE that the "Support" channel is private, and has the zendesk app added to it.