google-authenticator: 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/google-authenticator - from package: google-authenticator

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

On Debian possible default full paths are:

/usr/bin/google-authenticator - from package: libpam-google-authenticator

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

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