This usually means that the hub and lock and having trouble communicating which is caused by packet loss in the mesh. The smart locks app is a very basic app, it send a command to add or delete codes and if there no response from the lock (eg when there is packet loss) then it throws an error. It doesn’t retry or is designated to handle problematic issues. Try adding a buffering repeater between the lock and the hub which helps with mesh packet loss: FAQ: why would I need another beaming repeater if my zwave lock is already close to my hub? - #2 by RBoy
There is also a second issue you may be facing with your Schalge connect (and this issue is specific to Schalge Connect locks due to a firmware defect). Once a code is programmed it won’t reprogram the same code again. So if you have packet loss and the response was lost in the mesh you won’t be able to reprogram it using guest locks.
You can take a look at LUM which is much hardier and handles many of the problems with mesh packet loss, retries and tracking codes including creating advanced users: [RELEASE] Lock User Management (LUM)