pcl_png2pcd: 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/pcl_png2pcd - from package: pcl-tools

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

On Debian possible default full paths are:

/usr/bin/pcl_png2pcd - from package: pcl-tools

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

Error in other languages:
pcl_png2pcd: bevel nie gevind nie
pcl_png2pcd: comandă negăsită
pcl_png2pcd: comando não encontrado
pcl_png2pcd: commande introuvable
pcl_png2pcd: command not found
pcl_png2pcd: käsku ei ole
pcl_png2pcd: không tìm thấy lệnh
pcl_png2pcd: komanda nerasta
pcl_png2pcd: Kommando nicht gefunden.
pcl_png2pcd: kommandot finns inte
pcl_png2pcd: komut yok
pcl_png2pcd: nie znaleziono polecenia
pcl_png2pcd: níor aimsíodh an t-ordú
pcl_png2pcd: no se encontró la orden
pcl_png2pcd: no s'ha trobat l'ordre
pcl_png2pcd: opdracht niet gevonden
pcl_png2pcd: parancs nem található
pcl_png2pcd: perintah tidak ditemukan
pcl_png2pcd: príkaz nenájdený
pcl_png2pcd: příkaz nenalezen
pcl_png2pcd: команда не найдена
pcl_png2pcd: командата не е открита
pcl_png2pcd: 命令找不到
pcl_png2pcd: コマンドが見つかりません