Beginning January 13, 2020, a new rule will require that no one may take an FAA Knowledge Test without first obtaining an FTN. That is the FAA Tracking Number. If you already took your initial Knowledge Test you undoubtedly already have an FTN. It would have been assigned to you when you applied for your Remote Pilot certificate at IACRA.FAA.GOV.
When scheduling your FAA exam, the FTN will be a required piece of information. Ultimately, this will make delivery of your Remote Pilot certiciate a smoother operation.
Here is the bottom line. You must have an FTN to take any FAA Knowledge Test. This includes the Recurrent Test. But in that case, you already have an FTN. You simply must know what it is to provide it to the testing center.
If you have not yet taken an FAA Knowledge Test, you have to Register on IACRA and get an FTN. After taking your initial Knowledge Test, this is the same place you will go to apply for your Part 107 certificate. (There are no new certificates for the Recurrent Test. Just keep your Knowledge Test Report proving that you have taken the FAA exam to remain current.)
On IACRA, create an account as an "Applicant" and accept the "Terms of Service".
On the subsequent page, create your account including your full name (as it will appear on your FAA Airman Certificate) along with security questions (to recover your account) and a user ID and password.