So, on Manjaro 18, to install snap, you do the following

sudo pacman -Syu snapd 

Once installed you need to run a few commands to ensure it works as intended

sudo systemctl enable --now snapd.socket 

sudo ln -s /var/lib/snapd/snap /snap

sudo reboot 

If you have done the above then you should be able to install from snap

to test this you can use the following to install Visual Studio Code

sudo snap install code --classic

The first app installed after installing snap will require the snapd core is downloaded, so that will run first, followed by the app you have requested.