Slack users experiencing a channel_not_found error when sending messages via Incoming Webhooks to channels other than the default one often face issues related to channel visibility or permission constraints. This error commonly occurs if the webhook or associated bot user is not a member of the specified channel, especially if it's private. Solutions include ensuring the bot user is invited to the channel, verifying the channel name or ID, and checking bot permissions and scopes. Legacy webhooks might offer more flexibility, but newer Slack apps should consider using the chat.postMessage API for better control. Testing message delivery through Slack's API explorer can help diagnose issues by confirming the validity of authentication tokens and message formats. Knock's SlackKit offers a streamlined approach to managing Slack notifications, eliminating the need for manual token or bot membership management and providing tools for creating rich message templates and handling notifications across different channels efficiently.