ngram_test: 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/ngram_test - from package: speech-tools

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

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