Key-errors

Technically its usual thing sometimes ! Its not a deal breaker, but mayby for some?

reason to do sudo pacman -Syu after install is to make sure manjaro-system do its job.

This trouble-shoot is meaning to notify what you can do or could do.

1. check your time by : timedatectl status
2. reinstall manjaro-keyring and archlinux-keyring : sudo pacman -S manjaro-keyring archlinux-keyring

the first check if time is ok if time is bad, then you have to change your time or fix your time. because keyservers are connected. so time is importand to do:

sudo pacman-key --populate archlinux manjaro
sudo pacman-key --refresh-keys

installing manjaro-keyring or archlinux-keyring you will have that keys atleast.

if all dont work.

you have to reset your keys.

[remove] /etc/pacman.d/gnupg 
sudo pacman-key --init    (takes some time)
sudo pacman-key --populate archlinux manjaro
sudo pacman-key --refresh-keys

Further troubleshooting

https://wiki.archlinux.org/index.php/Pacman-key#Cannot_import_keys