show-installed: 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/show-installed - from package: yum-utils

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

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