pcl_nn_classification_example: 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/pcl_nn_classification_example - from package: pcl-tools

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

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