Buddy communicates with the Slack API via a dedicated app. While cloud accounts use the official Buddy app by default, on-premises users need to configure the application by hand. This is performed when adding the Slack integration for the first time, or from the 3-rd Party Apps tab in the admin settings of your instance.
Due to changes in Slack's authorization mode, applications created after February 24, 2021, are no longer able to communicate with Slack's API. With the release of Buddy v2.3.107, however, it is now possible to migrate your existing Slack app so that it supports the new authorization.