I’m new here. Four years ago I had a Z-Wave Kwikset 914 Smartcode lock installed to replace my front door lock by my home security company. They programmed my code for me, but I subsequently programmed a guest code without any difficulty. That was maybe 3-1/2 years ago. A year later I deleted that code. I now need to program a guest code, and after following the instructions to a T (same as last time), I have been unable to set the guest code. It keeps telling me “programming unsuccessful”. I don’t have a hub. I’m programming right at the lock. Can someone please help me?
You don’t have a hub? How does your zwave work? Are you only using it for its keypad?
If you have have a hub, the “Smart Lock Guest Access” app allows you to setup codes and share invites in the app. I think Kwikset also has their own app for a similar purpose.
Try resetting the lock by the mfg, instructions and trying again.
I’m sure that’s very frustrating. 
If you don’t have a hub, the codes are programmed directly into the lock, so you would need to get in touch with Kwikset support.