populations: 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

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/populations - from package: populations

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