treesize: 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/treesize - from package: prime-phylo

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

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