ngrams: 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/ngrams - from package: libtext-ngrams-perl

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

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