Why can I not re-register an existing Scanner Appliance?

An enrollment code can only be used once. It is valid for 1 hour and is revoked as soon as a Scanner Appliance registers with it, so you cannot use the same code to register an appliance again.

To register a Scanner Appliance again, generate a new enrollment code in Security Center and enter it in the appliance’s Probe registration menu.

Expired enrollement code?
If more than 1 hour has passed since you generated the code, it is no longer valid. Generate a new code and complete the registration within 1 hour.

Read more about the installation steps here:
How do I install a Scanner Appliance?