There are a few different reasons why this message appears:
Serial codes can only be registered once. If you have already registered the code using another account, you simply need to locate that account and login to it. The license(s) for your product(s) will be available in that account.
If you have forgotten the password, for each email you think you may have used, enter it under this Forgot Password link. See which one returns a login link from our system. Check any spam or trash folders, in case the recovery email ends up there. You can then use that link to regain access to the correct account.
If you can't remember which email you used, or have lost access to the email, please create a support ticket. Provide the team your 20-digit serial code, proof of ownership and your personal details, including mailing address and possible emails used.
Sometimes shops sell returned stock. In these cases someone has probably used the code and returned it. You will need to contact the store and request a refund or replacement, as we don't allow FL Studio license resale or transfer.
Unfortunately, sometimes people try to resell used codes on various auction or second-hand sites, in violation of EULA. We don't allow FL Studio license resale or transfer. We suggest you contact the seller to request a refund, or see what policies for 'Item not as described' or 'Faulty / not-working' goods exist on the site where you purchased.
Image Line Support Team