You are attempting manual activation on a machine without an internet connection and you run into the error message "All x of the entitlements for serial xxx-xxx-xxxxxx-xxxx have already been claimed". You will need to remove the current activation or remove yourself as the user in the customer portal first.
You're not prompted to log in with your Redgate ID when you manually activate so the licensing server counts the activation as an additional entitlement.
Manual activations will show in the customer portal as 'Device: (xxxx-xxxx-xxxx-xxxx)' and use up an entitlement.
You will need to repeat this each time you activate offline.