Installation Guide Troubleshooting
From ArchWiki
Installing the base system
Errors when running pacstrap
(PGP)Key "..." could not be imported
error: linux-api-headers: key "F990EB20FAEsomhing" is unknown
- Import PGP key EAE999BD, "Allan McRae <me@allanmcrae.com>", created on 2011-06-03? [Y/n]
error: key "Allan McRae <me@allanmcrae.com>" could not be imported
Can be due to incorrect date, to fix set the time manually or use ntpd -qg to use the network to set the time.(a case)
Key import fails otherwise
One solution is to change the core repo in /etc/pacman.conf from SigLevel = PackageRequired to SigLevel = Optional TrustAll But that would decrease security by a lot.