The customization cannot be installed because another version is currently installed...."

1. It means you have built the addin in debug on your machine and it is also installed from the deploy location.

2. Open “COM Add-ins” form by either

Going to the Developer ribbon and click on the “COM Add-ins” button

Or Open Excel File | Options and under “Add-Ins” select “COM Add-ins” and click “Go…”

3. Select “Excel Addin Script Help” and click the button “Remove”

4. Then you’ll need to uninstall and reinstall the Addin from "Programs and Features"