You need a self-compiled version of stm32flash as all the binaries of the Linux repos do not support STM32F722 MCUs. So compile yourself:

git clone https://git.code.sf.net/p/stm32flash/code stm32flash-code
cd stm32flash-code/
make
sudo make install

Then you can compile the MMDVM firmware for this board after installing the compiler:

sudo apt install gcc-arm-none-eabi
git clone --recurse-submodules https://github.com/g4klx/MMDVM.git
cd MMDVM
vi Config.h

The default Config.h for MMDVM_RPT_Hat is as follows:

#if !defined(CONFIG_H)
#define  CONFIG_H
#define EXTERNAL_OSC 12000000
#define MODE_LEDS
#if defined(STM32F1)
#define STM32F1_POG
#else
#define ARDUINO_DUE_ZUM_V10
#endif
#define STM32F4_NUCLEO_MORPHO_HEADER
#define MODE_PINS
#define SEND_RSSI_DATA
#define SERIAL_REPEATER
#define USE_DCBLOCKER
#endif

Build the MMDVM firmware for MMDVM_RPT_Hat:

make rpt_hat

And flash the firmware via locally compiled stm32flash:

sudo make deploy-rpt_hat

After flashing the board is automatically reset.