pstree.x11: 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.x11 - from package: psmisc

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

On Debian possible default full paths are:

/usr/bin/pstree.x11 - from package: psmisc

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

Error in other languages:
pstree.x11: bevel nie gevind nie
pstree.x11: comandă negăsită
pstree.x11: comando não encontrado
pstree.x11: commande introuvable
pstree.x11: command not found
pstree.x11: käsku ei ole
pstree.x11: không tìm thấy lệnh
pstree.x11: komanda nerasta
pstree.x11: Kommando nicht gefunden.
pstree.x11: kommandot finns inte
pstree.x11: komut yok
pstree.x11: nie znaleziono polecenia
pstree.x11: níor aimsíodh an t-ordú
pstree.x11: no se encontró la orden
pstree.x11: no s'ha trobat l'ordre
pstree.x11: opdracht niet gevonden
pstree.x11: parancs nem található
pstree.x11: perintah tidak ditemukan
pstree.x11: príkaz nenájdený
pstree.x11: příkaz nenalezen
pstree.x11: команда не найдена
pstree.x11: командата не е открита
pstree.x11: 命令找不到
pstree.x11: コマンドが見つかりません