There are a few reasons why you may not be seeing your upcoming guest's code on your smart lock.
Your lock is not linked to a listing. You must link your lock to a specific listing so that Host Tools knows which listing the lock is connected to and which guests need codes. See this article about linking a lock to a listing.
The reservation is more than 72 hours away (Seam locks). If you connected your lock via Seam.co, codes are only added to the lock within 72 hours of the guest's check-in time. If the reservation is more than 72 hours away, the code will appear closer to check-in.
The guest's phone number hasn't synced yet (last-minute bookings). For last-minute Airbnb bookings, the guest's phone number can take a minute or two to sync after the reservation is confirmed. Host Tools waits for the phone number to arrive before programming a Seam lock code — so the guest always receives the correct code. The code will appear on your lock automatically once the phone syncs. If check-in is approaching and no code has appeared after a few minutes, try refreshing the reservation or contact Airbnb support.