Sometimes Arch Linux complains about missing keys, after spending a little too long Googling around, this was the solution:
Upgrade archlinux-keyring first, then upgrade the rest of the system.
sudo pacman -Sy archlinux-keyring
If that doesn't solve your problems, try switching your keyserver
sudo nano /root/.gnupg/dirmngr_ldapservers.conf
keyserver line to:
as documented here: https://wiki.archlinux.org/index.php/Pacman/Package_signing#Cannot_import_keys
Your system time might be wrong as well, so remember to check that.
Sometimes when installing a tool, it says that a particular GPG key is outdated. This means that the tool is no longer maintained and a red flag. You might want to look for keys elsewhere.