To be able to build you install this package
sudo pacman -S archiso
Download also the Carli keyring from github and install it with pacman -U.
https://github.com/arcolinuxiso/carli_repo/tree/master/x86_64
Download the last version. Install with pacman -U nameofthefile
Or run the script import-carli-key.sh to download and sign it.
Do check out the archiso.readme.
Use the correct version of archiso.
Start building your own carli version with
sudo ./build.sh -v