“This serial key is already used on another PC” is shown and License cannot be activated, what should I do?

This error is shown when attempting to use a currently activated license. For instance, Computer-1 has an activated license but you install RLP5 on Computer-2 and attempt license activation.
In this case, deactivate the license on Computer-1 and then activate the license on Computer-2.

For the license deactivation, please refer to:
How can I deactivate the license of RasterLinkPro5?

Even when the computer would not boot, you can deactivate the license on the Internet by “Deactivation (When the PC is broken)” function if you have another computer which is connected to the Internet. For details, please refer to FAQ: How can I deactivate the license when the computer has broken down?