Fehler: Konnte den Vorgang nicht vorbereiten (Kann Abhängigkeiten nicht erfüllen)
:: nvidia-340xx: das Installieren von linux (4.12.10-1) verletzt Abhängigkeit von 'linux<4.12'

the same procedure as for kernel 4.11 some time ago

Please post the full log, not just a little snippet, so it will be visible which version your mirror has.
Did you update your install before trying to get this package? Did you check if your mirror is syncing?
nvidia-340xx in the KaOS repos needs linux>= 4.12

pacman -Sii nvidia-340xx
Repository      : main
Name            : nvidia-340xx
Version         : 340.102-14
Description     : NVIDIA drivers for linux.
Architecture    : x86_64
URL             : http://www.nvidia.com/
Licenses        : custom
Groups          : None
Provides        : None
Depends On      : linux>=4.12  linux<4.13  nvidia-340xx-utils=340.102
Optional Deps   : None
Required By     : None
Optional For    : None
Conflicts With  : nvidia  nvidia-304xx
Replaces        : nvidia-304xx
Download Size   : 4.53 MiB
Installed Size  : 4.68 MiB
Packager        : Anke Boersma <demm@kaosx.us>
Build Date      : Wed 30 Aug 2017 03:11:30 PM EDT
MD5 Sum         : 8ac45a0b1f9521054f9217e869b94cbd
SHA-256 Sum     : a3721bd6476c0464369561365ebb1791fc9e125747c63d5a3ae67b3bc83f1ddd

    demm
    update 10.09.17 has corrected the error