A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.
When we start something new, we likely have grand aspirational plans for what it one day will be. Through rose-coloured glasses we see its potential.
Dreaming big is fine. Having big plans is fine. Starting off with the most complex system possible, however, is inviting failure to your doorstep.
What's the smallest piece you can work on today that will produce some value? Start with that. Then find the next smallest piece, and add that. Rinse. Repeat.
Starting simple doesn't guarantee that your project or objective will succeed. Note that Gall doesn't state that. What he does say is that starting complex will fail.