Hi!
Try this
sudo pacman -Sy archlinux-keyring manjaro-keyring
sudo pacman-key --populate archlinux manjaro
sudo pacman-key --refresh-keys
If doesn’t work, try this
Delete package cache
pacman -Sc
Delete as root
/etc/pacman.d/gnupg
Regenerate it with
sudo pacman-key --init
sudo pacman-key --populate archlinux