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
A flat mount disc caliper on a post mount frame? There's an adapter for that - CyclingTips
Direct mount to flat mount adapter Kinning Cycles - YouTube