You need to set up your Arch install to be able to build packages.
See instructions for that: https://wiki.archlinux.org/title/Makepkg#Usage
If you have git installed, you can just clone the whole repository.
git clone https://github.com/UltimMC/ultimmc-pkgbuild.git
cd ultimmc-pkgbuild
makepkg -si
If you do not have git installed, you can use a downloader like wget or directly download the PKGBUILD using your browser.
mkdir ultimmc-pkgbuild
cd ultimmc-pkgbuild
wget https://raw.githubusercontent.com/UltimMC/ultimmc-pkgbuild/master/PKGBUILD
makepkg -si
Report issues here.