Uninstall Taskfeed: Considerations for External Sharing
Removing the External Sharing Permission Assignment
“I'm unable to uninstall the Taskfeed package. I'm receiving the below errors. The Permission Set has no assigned users, so I'm not sure why I'm getting that error. Additionally, I'm not able to remove the Taskfeed Site from Salesforce.”
The permission set referenced above has been assigned to the Site Guest User profile, for use with External Sharing and stored in Sites.
This profile is found in Setup > Sites under the Taskfeed External Sharing site configured in your Org.
To remove the permission set:
From Setup, enter Sites in the Quick Find box, then select Sites.
Select the Taskfeed Public Shares Services site configured for External Sharing.
Click Public Access Settings.
Next, click View Users
Select the Site Guest User.
Navigate to the Permission Set Assignment section and delete the assigned permission.
Scroll down the page to the Managed Package section and Remove Taskfeed.
Go back to Sites and Deactivate the Taskfeed force.com site
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For more information: Managing Salesforce Sites
You will also have to select a different page there in the Active Site Home page lookup so it's not associated with Taskfeed. Just select any other page.
Once that's done, you should be able to uninstall successfully.