This error indicates that there is still an active PlanSwift process running, and the installer cannot replace the existing file because it is in use. The most common scenario would be that the PlanSwift program did not close during an update.
If you see PlanSwift on the application bar at the bottom of your screen, simply right-click on it and select Close.
Next press Retry on the error message box. If the error persists, restart your computer, then run PlanSwift and the update it before running any other applications.