Canvas admins can enable the Slack feature option for a Canvas account. However, to enable Canvas notifications for all users within a Slack workspace, a user with Slack admin rights must create and install the Slack app to create a Bot Access Token. After the bot is created and verified, individual Canvas users can enable Slack as a Canvas contact method to receive notifications.
Enable Slack Feature Option
In Canvas, click the Settings link  in Account Navigation. Then click the Feature Options tab . Enable the Slack Notifications feature option .
Open Slack Apps
To configure Slack for all users in a Slack workspace, a Slack app and API key must be created.