You can't enter a key that has already been registered.
Follow this guide from step two: https://support.presonus.com/hc/en-us/articles/360003305131--3-Studio-One-4-Software-Activation-Online-Computer-
If the issue remains, please create a ticket to technical support.
Here is a link to create a ticket: http://support.presonus.com/hc/en-us/requests/new