Even if you are connected to the internet - a new installation is likely to require a sync of metadata (database) from a current mirror - up-to-date that is
sudo pacman-mirrors --continent
Then sync your system
sudo pacman -Syu
If you do not have internet connection
- theres is guide to some basic troubleshooting
→ [root tip] [How To] Basic network know-how and troubleshooting
For a user new to Manjaro world
- another topic worth visiting is
→ [root tip] [How To] Update Manjaro the smart way