Composer Package Manager Horde 6 needs the Composer Package Manager.
Currently, composer 2.2 - 2.4 are supported. Preferred: Get it via Phar.io Phive Installer Install only for you as a normal user

phive install composer
OR Install for all users as root into /usr/local/bin

phive install composer
See https://phar.io/#Install Manual download See https://getcomposer.org/download/ for instructions. Place the downloaded file in /usr/local/bin or another location in the search path and set the executable bit OpenSUSE 15.4 and Tumbleweed

sudo zypper in php-composer2
Ubuntu 22.04 and up

sudo apt install composer
Fedora, RedHat, RHEL See Phar.io Phive or manual download Debian 11 See Mailing list:
https://marc.info/?l=horde\&m=167152123830875\&w=2 (or see Manual download above) Debian 12

sudo apt install composer