Isso é quase sempre causado pelo fato de sua rede não estar configurada ou conectada, o que pode ser causado por erro do usuário, como não conectar o cabo ou um driver ausente para algumas placas wifi.
Estou usando o manjaro KDE e tenho esse problema em que não consigo atualizar o sistema, obter qualquer arquivo do pacman enquanto os espelhos estão on-line e estou conectado à Internet.
$ pacman -Fs
warning: database file for 'core' does not exist
warning: database file for 'extra' does not exist
warning: database file for 'community' does not exist
warning: database file for 'multilib' does not exist
warning: database file for 'blackarch' does not exist
error: no targets specified (use -h for help)
$ sudo pacman -Syu
:: Synchronizing package databases...
error: failed retrieving file 'core.db' from manjaro.barata.pt : The requested URL returned error: 404
error: failed retrieving file 'core.db' from manjaro.telecoms.bg : The requested URL returned error: 404
error: failed retrieving file 'core.db' from mirror.ventraip.net.au : The requested URL returned error: 404
error: failed retrieving file 'core.db' from ftp.halifax.rwth-aachen.de : The requested URL returned error: 404
error: failed retrieving file 'core.db' from repo.sadjad.ac.ir : The requested URL returned error: 404
error: failed to update core (unexpected error)
error: failed retrieving file 'extra.db' from manjaro.barata.pt : The requested URL returned error: 404
error: failed retrieving file 'extra.db' from manjaro.telecoms.bg : The requested URL returned error: 404
error: failed retrieving file 'extra.db' from mirror.ventraip.net.au : The requested URL returned error: 404
error: failed retrieving file 'extra.db' from ftp.halifax.rwth-aachen.de : The requested URL returned error: 404
error: failed retrieving file 'extra.db' from repo.sadjad.ac.ir : The requested URL returned error: 404
error: failed to update extra (unexpected error)
error: failed retrieving file 'community.db' from manjaro.barata.pt : The requested URL returned error: 404
error: failed retrieving file 'community.db' from manjaro.telecoms.bg : The requested URL returned error: 404
error: failed retrieving file 'community.db' from mirror.ventraip.net.au : The requested URL returned error: 404
error: failed retrieving file 'community.db' from ftp.halifax.rwth-aachen.de : The requested URL returned error: 404
error: failed retrieving file 'community.db' from repo.sadjad.ac.ir : The requested URL returned error: 404
error: failed to update community (unexpected error)
error: failed retrieving file 'multilib.db' from manjaro.barata.pt : The requested URL returned error: 404
error: failed retrieving file 'multilib.db' from manjaro.telecoms.bg : The requested URL returned error: 404
error: failed retrieving file 'multilib.db' from mirror.ventraip.net.au : The requested URL returned error: 404
error: failed retrieving file 'multilib.db' from ftp.halifax.rwth-aachen.de : The requested URL returned error: 404
error: failed retrieving file 'multilib.db' from repo.sadjad.ac.ir : The requested URL returned error: 404
error: failed to update multilib (unexpected error)
blackarch is up to date
:: Starting full system upgrade...
resolving dependencies...
looking for conflicting packages...
Packages (156) ark-17.12.1-1 attica-qt5-5.42.0-1 baloo-5.42.0-1
baloo-widgets-17.12.1-1 beef-1:2993.521e5c3e-1
bluez-qt-5.42.0-1 breeze-icons-5.42.0-1 cantata-2.2.0-3
dolphin-17.12.1-1 dolphin-plugins-17.12.1-1 ffmpeg-1:3.4.1-3
ffmpegthumbs-17.12.1-1 filelight-17.12.1-1
frameworkintegration-5.42.0-1 freetype2-2.9-1 glib2-2.54.3-2
gst-plugins-bad-1.12.4-2 gst-plugins-ugly-1.12.4-3
gtk2-2.24.32-1 gvfs-1.34.1+11+gad63dfb7-1
gvfs-afc-1.34.1+11+gad63dfb7-1
gvfs-gphoto2-1.34.1+11+gad63dfb7-1
gvfs-mtp-1.34.1+11+gad63dfb7-1 gvfs-nfs-1.34.1+11+gad63dfb7-1
gvfs-smb-1.34.1+11+gad63dfb7-1 gwenview-17.12.1-1 inxi-2.3.56-1
k3b-1:17.12.1-1 kaccounts-integration-17.12.1-1
kaccounts-providers-17.12.1-1 kactivities-5.42.0-1
kactivities-stats-5.42.0-1 kamera-17.12.1-1 karchive-5.42.0-1
kate-17.12.1-1 kauth-5.42.0-1 kbookmarks-5.42.0-1
kcalc-17.12.1-1 kcmutils-5.42.0-1 kcodecs-5.42.0-1
kcompletion-5.42.0-1 kconfig-5.42.0-1 kconfigwidgets-5.42.0-1
kcoreaddons-5.42.0-1 kcrash-5.42.0-1 kdbusaddons-5.42.0-1
kdeclarative-5.42.0-1 kdeconnect-1.2.1-1 kded-5.42.0-1
kdegraphics-thumbnailers-17.12.1-1 kdelibs4support-5.42.0-1
kdenetwork-filesharing-17.12.1-1 kdesu-5.42.0-1
kdialog-17.12.1-1 kdnssd-5.42.0-1 kdoctools-5.42.0-1
keditbookmarks-17.12.1-1 kemoticons-5.42.0-1
kfilemetadata-5.42.0-1 kfind-17.12.1-1 kget-17.12.1-1
kglobalaccel-5.42.0-1 kguiaddons-5.42.0-1 khelpcenter-17.12.1-1
kholidays-17.12.1-1 khtml-5.42.0-1 ki18n-5.42.0-1
kiconthemes-5.42.0-1 kidletime-5.42.0-1 kimageformats-5.42.0-1
kinit-5.42.0-1 kio-5.42.0-1 kio-extras-17.12.1-1
kirigami2-5.42.0-1 kitemmodels-5.42.0-1 kitemviews-5.42.0-1
kjobwidgets-5.42.0-1 kjs-5.42.0-1 kjsembed-5.42.0-1
knewstuff-5.42.0-1 knotifications-5.42.0-1
knotifyconfig-5.42.0-1 konsole-17.12.1-1 kpackage-5.42.0-1
kparts-5.42.0-1 kpeople-5.42.0-1 kpty-5.42.0-1
krunner-5.42.0-1 kservice-5.42.0-1 ksystemlog-17.12.1-1
ktexteditor-5.42.0-1 ktextwidgets-5.42.0-1 kuiserver-5.11.5-2
kunitconversion-5.42.0-1 kwallet-5.42.0-1
kwalletmanager-17.12.1-1 kwayland-5.42.0-1
kwidgetsaddons-5.42.1-1 kwindowsystem-5.42.0-1 kxmlgui-5.42.0-1
lib32-mesa-17.3.2-2 lib32-mesa-vdpau-17.3.2-2
lib32-systemd-236.81-1 libcdio-2.0.0-1
libcdio-paranoia-10.2+0.94+2-2 libkcddb-17.12.1-1
libkdcraw-17.12.1-1 libkexiv2-17.12.1-1 libkipi-17.12.1-1
libksane-17.12.1-1 libnm-1.10.3dev+38+g78ef57197-1
libnm-glib-1.10.3dev+38+g78ef57197-1 libpagemaker-0.0.4-1
libqalculate-2.2.1-1 librsvg-2:2.42.0-1 libseccomp-2.3.2-2
libsoup-2.60.3-1 libsrtp-1:2.1.0-1 libsystemd-236.81-1
libva-mesa-driver-17.3.2-2 libwebp-0.6.1-1
libx264-2:152.20171224-1 libx264-all-2:152.20171224-1
linux414-4.14.14-1 linux414-nvidia-1:384.111-3
manjaro-release-17.1.2-1 mesa-17.3.2-2 mesa-vdpau-17.3.2-2
mhwd-0.6.1-1 mhwd-db-0.6.1-1 modemmanager-qt-5.42.0-1
mpd-0.20.15-2 networkmanager-1.10.3dev+38+g78ef57197-1
networkmanager-qt-5.42.0-1 okular-17.12.1-1
oxygen-icons-1:5.42.0-1 perl-xml-libxml-2.0132-1
plasma-framework-5.42.0-1 plasma-workspace-5.11.5-2
polkit-0.113+34+g29ba7af-1.1 potrace-1.15-1
print-manager-17.12.1-1 prison-5.42.0-1
qqc2-desktop-style-5.42.0-1 signon-kwallet-extension-17.12.1-1
solid-5.42.0-1 sonnet-5.42.0-1 spectacle-17.12.1-1
syntax-highlighting-5.42.0-1 systemd-236.81-1
systemd-sysvcompat-236.81-1 threadweaver-5.42.0-1 tslib-1.15-1
vcdimager-2.0.1-1 vlc-nightly-3.0.0v20180114-1 yakuake-3.0.4-2
Total Download Size: 297.51 MiB
Total Installed Size: 960.03 MiB
Net Upgrade Size: 6.49 MiB
:: Proceed with installation? [Y/n]
A ajuda seria apreciada.
Isso é quase sempre causado pelo fato de sua rede não estar configurada ou conectada, o que pode ser causado por erro do usuário, como não conectar o cabo ou um driver ausente para algumas placas wifi.