pstree: 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/pstree - from package: psmisc

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

On Debian possible default full paths are:

/usr/bin/pstree - from package: psmisc

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

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