Hello marvin,
It's important that in your application you have the flash_config files. These files include the flash configuration block that the ROM bootloader will utilize to communicate with your external memory. You can take as a base the files that we provide within the RT685-EVK SDK, but you will need to make the proper changes to adapt those to the memory that you are using.
Besides that, you need to verify that you have selected a driver so that the debugger can program the external flash. You can see this in the MCU settings on the properties of your project.
victorjimenez_0-1624029869574.png
Also, in the preprocessor settings of your project, it's important to verify that you declared the BOOT_HEADER_ENABLE symbol to 1.
victorjimenez_1-1624029955695.png
If you connect the memory to FlexSPI port B, you must configure the ISP pins properly.
victorjimenez_2-1624030024738.png
Regards,
Victor