content-object-add: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/content-object-add - from package: php-horde-content

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/content-object-add - from package: php-horde-content

You may install the required package with command apt-get install {package}

Error in other languages:
content-object-add: bevel nie gevind nie
content-object-add: comandă negăsită
content-object-add: comando não encontrado
content-object-add: commande introuvable
content-object-add: command not found
content-object-add: käsku ei ole
content-object-add: không tìm thấy lệnh
content-object-add: komanda nerasta
content-object-add: Kommando nicht gefunden.
content-object-add: kommandot finns inte
content-object-add: komut yok
content-object-add: nie znaleziono polecenia
content-object-add: níor aimsíodh an t-ordú
content-object-add: no se encontró la orden
content-object-add: no s'ha trobat l'ordre
content-object-add: opdracht niet gevonden
content-object-add: parancs nem található
content-object-add: perintah tidak ditemukan
content-object-add: príkaz nenájdený
content-object-add: příkaz nenalezen
content-object-add: команда не найдена
content-object-add: командата не е открита
content-object-add: 命令找不到
content-object-add: コマンドが見つかりません