The licence key has already been activated - message is seen

It will happens due to the following reasons,

1. If the license key is already activated in another system.
2. The Licensing Server looks for computer ID only, If the computer ID changed it won't allow to activate. The following situations can result in a changed Machine ID:

• When you've upgraded your OS, your computer ID has been changed even though the computer name remains same
• When the network adapter changed.
• When the motherboard has been replaced.
• When network card has been replaced.
• When running AccountEdge in a Virtual Machine (such as Virtualbox or VMWare).
• The way you connect to the network is a USB modem or express card. If you connect this way your Machine ID may only remain the same for that particular connection instance.
• On Windows some network adapters allow MAC address changing as one of the properties in the adapter's advanced settings. You can find these settings in the Device Manager.