I do not think I have ever managed to start the APU, and as far as I remember, the electronics page doesn’t work in the flight computer, so I’ve always assumed it is a bug or not yet implemented. While this is not an ideal situation you should be able to use Ctrl+E to start your engines.
I have another issue I am going to try and repo for someone today involving the B78X, I’ll try to start my APU as well. I have bookmarked this thread, but if you don’t have a response from me by this time tomorrow please ping me to remind me.