To uninstall ClosePlan app from a Sandbox or a Production Org, the following are required:

Uninstall ClosePlan:

  1. Go to ClosePlan Admin tab > Environment > Maintenance > hit Delete Context Metadata button and Confirm

2. Go to Setup > Historical Trending > object CP Event > disable the checkbox Enable Historical Trending and Save

3. Go to Installed Packages and uninstall ClosePlan app

Troubleshooting:

If Delete Context Metadata button is disabled: