Hello, here I post all the commands that were very handy when you get the error mentioned in the headline. I am not an expert and I found this commands in manjaro forum which is not availiible right now. So I post these commands.
sudo pacman -Sy archlinux-keyring manjaro-keyring
sudo pacman-key --populate archlinux manjaro
sudo pacman-key --refresh-keys
sudo pacman -Syu
Apply these commands and your problem should be resolved. If it does not work, maybe there are some corrupt package files which can be deleted by this command,
sudo rm /var/cache/pacman/pkg/*.part