In order to send leads to your private channels, it is required that the person who initiated the integration is member on the private channel.
You can see who that member is by accessing your Slack settings in Leadfeeder (here's how to get to those settings) and seeing who is the Integration admin.
If you want to let Leadfeeder to post to a private channel
- Make sure that the Integration admin user is added to the private channel.
- Get back to Slack settings and click on Sync from Slack to get the channel available.
After that the private channel will be available while sending single leads to your Slack workspace or when you are setting Leadfeeder to notify you automatically about leads filling specific criteria via Slack.