For me it solved the error.
Thank you very much.

Hello,
I'm also receiving this message recently
GPG error: https://debian.qelectrotech.org/qet/builds/debian stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B8D8B0B00ADD2851

System:
  Kernel: 6.12.12+bpo-amd64 arch: x86_64 bits: 64 compiler: gcc v: 12.2.0
  Desktop: Cinnamon v: 5.6.8 tk: GTK v: 3.24.38 wm: Muffin dm: LightDM Distro: Debian GNU/Linux
    12 (bookworm)

  Active apt repos in: /etc/apt/sources.list.d/qelectrotech.list
    1: deb [signed-by=/etc/apt/trusted.gpg.d/Qelectrotech_Repository.asc] https: //debian.qelectrotech.org/qet/builds/debian stable main