checkstyle: 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/checkstyle - from package: checkstyle

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

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