Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32H742x/MDMA/MDMA_C8TBR#0x0
MDMA channel x Trigger and Bus selection Register
Trigger selection
Source BUS select This bit is protected and can be written only if EN is 0.
Destination BUS slect This bit is protected and can be written only if EN is 0.
https://github.com/cmsis-svd/cmsis-svd-data