OK, I’m gonna guess it expects Synaptic to be part of the ISO image … so if you’re using Kubuntu as the base ISO for your custom build you’re stuck.
either get hold of Remastersys and try that, learn to do it manually … or erm, give up 
I’ll have to try and fix my dual boot setup as I have 2 Ubuntu, 1 Kubuntu and a manjaro OS set up and the only one that works is Kubuntu, I can’t remember if Manjaro works as I tried it and found I don’t like it so have hardly used it.
Thinking about it, maybe using Kubuntu as a base isn’t the best plan. Linux Mint KDE uses Ubuntu as a base, then adds in KDE - I always thought this strange, but maybe it’s to get around exactly these issues