I managed to get this working but it was a pain.
First, upgrade to 2.5.1 firmware on the C8 if you haven't yet.
Next exclude or factory reset the Schlage FE599 lock.
Make sure in the C8 Pro under Settings -> Z-Wave Details -> the table of devices at the bottom: That you don't have the device half added...If it is, you might want to exclude it again, remove it, and factory reset the lock again.
Go through Settings -> Shutdown and wait for it to tell you it is safe to unplug it. Takes 1-2 minutes. Unplug the C8 Pro for 2 minutes and let the zwave chip drain.. Google AI says there is a cache on the chip that needs to be flushed. I just know that I was having a hell of a time adding Schlage Locks until I did this step and it immediately worked on the next try.
After 2 minutes off, plug the C8 Pro back into power and let it boot up for at least 2 minutes.
Pair the Schlage lock again. I used Z-Wave Pairing. The lock gets detected but the pairing ends with an error because it didn't complete. Don't give up here. Go straight to the Settings -> Z-Wave Details again and you should see the device there. Refresh it. It should refresh (once or twice) and detect it as a Generic Lock.
Once it has it as a generic lock, go to Devices -> the FE599 you just added (it's going to have a screwed up name because the pairing didn't go through the final steps)
Go to the Device Info tab of the lock. Set the Device label to whatever you want to call the lock in your dashboard. Set the Device name to Schlage FE599 Lock. Set the Type from the drop down to Schlage FE599/BE369 Lock. Turn on command retry. Save this and go to the commands tab.
On commands you need to set the pin code length to 4. That's what the lock supports but I have a sneaky suspicion that is one of the things that doesn't get initialized properly in the current pairing procedure.
After you set the pin length to 4, you need to refresh the status of the lock on that same screen. Try pulling the default codes from the lock. Down in the current states you should see the 2 codes but probably with stars.. if you get no response at all, go back to Settings - Z-Wave Details and refresh the new lock... Update neighbor nodes. Get it talking. I had to refresh mine 2 or 3 times to get a normal status on that screen.
When I went back to commands I was able to get 1 code on one lock and 2 codes on the other.
Delete the default codes. If you rush this and get impatient it'll just jam up. After you refresh the lock on the details screen it should start working over here, but it might only work for one command or two before you have to refresh it again on the settings / zwave details again and then back to command on the new lock.
After setting the first few pin codes the lock started to "talk" to the hub better.
For this whole programming process I'm using a USB power bank and I have my C8 Pro connected to my WiFi so I can take it to the locks. Or I take take the locks off the door and put them next to the C8 Pro. Mine are usually less than 1' away when pairing and when setting up the initial codes.
I have a range extender but it was the last thing that I paired. I paired the FE599 locks first.
I had a hell of a time with these locks on Vera3 12 years ago and VeraSecure.
Right now I'm having trouble getting some BE469's to factory reset. I'm probably rushing a step like not waiting 10 minutes for the battery to drain. Hard to be patient in the heat being eaten by mosquitoes.
I hope the info I gave here helps some people overcome difficulties with their FE599's. It can can be done and once done it seems to be working so far. I think the two key things are to configure first as a generic lock type and reconfigure as the schlage FE599 type. I think this is getting the lock close to initialized and with some refreshing the controller and lock relationship becomes healed. The second issue is this lock cannot be programmed as fast as the BE468's that I did get working.
If you can't get it working this way you might want to try the Switch to ZWaveJS that is on Settings -> ZWave Details. I haven't tried it but I read that people have gotten their locks paired with that. I was already so far along in my setup and finally getting somewhere with my locks that I didn't give that a try yet.