opam-installer: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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-installer - from package: opam

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