camlp4r.opt: 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/camlp4r.opt - from package: ocaml-camlp4-devel

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

On Debian possible default full paths are:

/usr/bin/camlp4r.opt - from package: camlp4

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

Error in other languages:
camlp4r.opt: bevel nie gevind nie
camlp4r.opt: comandă negăsită
camlp4r.opt: comando não encontrado
camlp4r.opt: commande introuvable
camlp4r.opt: command not found
camlp4r.opt: käsku ei ole
camlp4r.opt: không tìm thấy lệnh
camlp4r.opt: komanda nerasta
camlp4r.opt: Kommando nicht gefunden.
camlp4r.opt: kommandot finns inte
camlp4r.opt: komut yok
camlp4r.opt: nie znaleziono polecenia
camlp4r.opt: níor aimsíodh an t-ordú
camlp4r.opt: no se encontró la orden
camlp4r.opt: no s'ha trobat l'ordre
camlp4r.opt: opdracht niet gevonden
camlp4r.opt: parancs nem található
camlp4r.opt: perintah tidak ditemukan
camlp4r.opt: príkaz nenájdený
camlp4r.opt: příkaz nenalezen
camlp4r.opt: команда не найдена
camlp4r.opt: командата не е открита
camlp4r.opt: 命令找不到
camlp4r.opt: コマンドが見つかりません