Thanks for reaching out to Jotform Support. When configuring the integration, you must first select one channel. After you've set the fields that you want to include in the message, click save, and you'll be given the option to add more channels. Let me show you how:
In Form Builder, in the orange navigation bar at the top of the screen, click on Settings.
Click on Integrations on the left side of the screen.
Select the Slack Integration.
After setting up the other options, click on Save.
Click on Add New, select a channeland Complete the Integration.
That's it. Let us know if you have any other questions.