A flat mount disc caliper on a post mount frame? There's an adapter for that - CyclingTips
Amazon.com: SwishTi Cyclocross Gravel E-Bike Bicycle Bike Flat-Mount Disc Brake Adapter for Flat-Mount Forks and Post-Mount Calipers use w/2pcs FM Bolts : Sports & Outdoors
Flat Mount Disc Brake Adaptors - Velo Orange
Amazon.com : SHIMANO Front 160 Rotor, Post Mount Caliper to Flat Mount Fork : Sports & Outdoors
Shimano Flat Mount Disc Brake Adapter - Components