update-ca-certificates: 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
/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/sbin/update-ca-certificates - from package: ca-certificates

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