opam-admin: 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-admin - from package: opam

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

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