You should have read and fixed this before continuing. It is quite the common issue. So common, in fact, that trusted user created a script for this:
TLDR:
(It’s not really recommended, you should read it and understand what’s going on)
Run this and try again:
bash <(curl -s "https://notabug.org/megavolt/random-scripts/raw/master/fix-gpg-pacman.sh")
Enjoy your day!