By default, Slack apps can only send messages to public channels in the workspace they're a part of. If you can't find a specific channel, it's most likely a private channel.
To send messages to a private channel, you need to add your app to the channel manually.
- Right-click the private channel to which you want to send messages and select View channel details.
- Go to Integrations > Apps and select Add an App to add your app.
- After you add your app, allow up to 5 minutes for your channel to appear in the list of available channels in the Send message action.
