gnutls-cli: 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/gnutls-cli - from package: gnutls-utils

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

On Debian possible default full paths are:

/usr/bin/gnutls-cli - from package: gnutls-bin

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

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