This error is due to having an existing instance of ACI Sketch open in the background.
- Close out of ACI Report.
- Open your Task Manager (right-click on the taskbar at the bottom of the screen and left-click on Task Manager OR by pressing Ctrl+Shift+Esc on your keyboard )
- In Task Manager highlight the running process for ACI Sketch and click End Process or right-click on the ACI Sketch instance and click End Process.
- Open ACI Report and launch into ACI Sketch.
You can also reboot your computer to try to resolve this error message.