psi: 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 Debian possible default full paths are:

/usr/bin/psi - from package: psi

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

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