Since mojo needs a version of ncurses that is not available on normal way on Arch, you need to do some extra steps to get mojo working.
python <(curl -sSL https://raw.githubusercontent.com/Sharktheone/arch-mojo/main/install.py)
Install mojo globally:
python <(curl -sSL https://raw.githubusercontent.com/Sharktheone/arch-mojo/main/install.py) --global
Change working directory:
python <(curl -sSL https://raw.githubusercontent.com/Sharktheone/arch-mojo/main/install.py) --dir=/tmp/arch-mojo
First install modular with the official instructions Modular
python <(curl -sSL https://raw.githubusercontent.com/Sharktheone/arch-mojo/main/install.py) --fedora