pngtoexr: 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/pngtoexr - from package: exrtools

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

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