opam: 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/opam - from package: opam

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

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