Sonoma won't boot on my MBP mid-2015 without flash drive

Hello everyone. Sorry if it's a really stupid question, but why can't I boot into the system and get this prohibitory sign if I eject my flash drive?

I've just installed Sonoma on my MBP mid-2015, applied root patching and it works fine as long as the flash drive is inserted, but as soon as it's ejected I get this.