clang-modernize: 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/lib/llvm-3.4/bin/clang-modernize - from package: clang-modernize-3.4
/usr/lib/llvm-3.5/bin/clang-modernize - from package: clang-modernize-3.5

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

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