Hi there,
You should probably go and watch the Video on adding a build configuration (youtube) I don’t see yours? why only System builds? Did you create a New Board ?
Let me switch toolchains and get back, Probably because there’s no board defined in the BUILD ? 'm guessing or its just a copy of the DK system build?? either way it’s not what you want. Are you using a Xiao Dev board ? Is it configure the same as what I posted ?
So if you power off the Xiao Dev Board does the NRF52840 change to _xxAA_REV3 ?
when you refresh the dev board?
Start with the basic’s doesn’t matter if you can flash it , as that’s going to be required to debug anyway. 
When you create the build .conf do you see it find the device board file? “Xiao_ble”
LMK
HTH
GL 
PJ 
the more I look at this , If you can debug the DK builds, Then the answer is in between the build files and the Board files.
Also besure the toolchain MATCHES the SDK version…
Here is the VB About info , What’s is yours?
Version: 1.96.2 (user setup)
Commit: fabdb6a30b49f79a7aba0f2ad9df9b399473380f
Date: 2024-12-19T10:22:47.216Z
Electron: 32.2.6
ElectronBuildId: 10629634
Chromium: 128.0.6613.186
Node.js: 20.18.1
V8: 12.8.374.38-electron.0
OS: Windows_NT x64 10.0.26100