protoc: 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/protoc - from package: protobuf-compiler

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

On Debian possible default full paths are:

/usr/bin/protoc - from package: protobuf-compiler

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

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