The following Slack error appears for a user in the Slack Workspace after adding the Salesforce for Slack app to the Slack Workspace:
Note: This article is specific to the Salesforce for Slack app. See Identify Salesforce Slack Apps and Support to verify exactly what Salesforce and Slack app your organization is using.
Prerequisite Salesforce Setup for Salesforce for Slack:
If the above setup and configuration is complete, this error is also known to occur if there is a lingering or defunct Slack mapping to a refreshed over or deleted sandbox or expired developer or trial org.
It's recommended and best practice to instruct users to remove Slack user mappings and remove the app once testing has been completed in non-production environments. Use the instructions to Disconnect Slack from Salesforce.
If you still have access to the user's connected Salesforce environment, instruct impacted users to log in and remove their Slack User Mappings. Unfortunately, this must be done by each user individually logging in to the Salesforce org and removing their own mappings.
Please consider promoting the following Idea for admins to centrally manage Slack user mapping with a future release: Mass deploy and authenticate Salesforce to Slack connections.
If you no longer have access to the connected Salesforce environment, log a case with Salesforce Support to investigate whether the user mapping may need to be manually removed. When logging a case, ask Support to reference the internal information section of this article for next steps and provide the following details to help expedite the case's investigation:
001295888

We use three kinds of cookies on our websites: required, functional, and advertising. You can choose whether functional and advertising cookies apply. Click on the different cookie categories to find out more about each category and to change the default settings.
Privacy Statement
Required cookies are necessary for basic website functionality. Some examples include: session cookies needed to transmit the website, authentication cookies, and security cookies.
Functional cookies enhance functions, performance, and services on the website. Some examples include: cookies used to analyze site traffic, cookies used for market research, and cookies used to display advertising that is not directed to a particular individual.
Advertising cookies track activity across websites in order to understand a viewer’s interests, and direct them specific marketing. Some examples include: cookies used for remarketing, or interest-based advertising.