If you receive an error "The slack resource “example-channel” does not exist or has not been granted access in the “example-workspace” Slack workspace" while trying to add an alert rule to a channel, we recommend checking two things.
1. Confirm the Sentry app is installed within your workspace. In your Slack channel you can click the 'i' in a circle icon >> 1 App (or 2 Apps, etc) to see apps installed on that channel.
2. If the Slack channel was once private and then turned public, we recommend sending the alerts to a channel that has always been private or creating a new private channel.
Slack - Can't add alert rule to channel
