This is for v. 38.5, but I don't know if it has been fixed. I found out recently that the unit Guide obsolete relatively soon after it is unlock and a good while before its replacement becomes available. Specifically it is discovered at exploration, an early ancient tech, obsoleted at writing, an mid ancient tech, and its replacement becomes available at road building, an early classical tech. Since Guide replace the earlier types of scouts, then we get a periode of about a half to three quarters of an age where you cannot build any scouts. I looked into the code, and Guide has a force obsoletion at writing, which I commented out in my version, which got it to work.