I am currently testing deployment of the MS 365 apps. But we are running into the following error when users are trying to activate the MS 365 apps: Another account from your organization is already signed in on this computer. Enter a product key instead. This happens when users are already signed into older Office versions with their old User Principale Name (UPN) and it was changed was changed before the installation of the MS 365 apps. In this post I will explain the issue and show the solution to prevent this issue and to fix it when you have got the activation issue.