Flashing CubeOrange plus

I’m trying to flash CubeOrange plus with ST link, but getting the errors “Error: Failed to erase memory” and others as shown in the screenshots attached.
Can someone help me with resolving this?

Use Mission Planner or QGC to flash. The Cube should come with a bootloader capable of receiving firmware from either. A lower level programmer requires DFU mode, which cannot be accessed without unnecessary disassembly.

Thanks @Yuri_Rage for the reply.
We are using custom signed arducopter bootloader - firmware combination. So we need to flash the bootloader as well which is where the issue is right now. Is there any flags to be set to enable flashing?