iLok activation codes from Synchro Arts are single-use redemption codes which place a license into an iLok account.
If a code has already been used, the redemption limit error will be displayed every time it is entered.
Instead of entering the code again, the existing license's activations must be managed from the iLok account it was redeemed into, using the iLok License Manager application.
Please see iLok's FAQ for help with performing license activations:
https://help.ilok.com/faq_licenses.html#activate_license
If you need to deactivate a license from an inaccessible computer so that it can be placed on a new location, you will want to make a machine activation reset request, also performed in the iLok License Manager, when logged into the correct account:
https://help.ilok.com/faq_licenses.html#machine_reset