latexindent: 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/latexindent - from package: texlive-extra-utils

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

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