pkcrop: command not found

Have You tried accessing the binary with the full path?

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/pkcrop - from package: pktools

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