raxmlHPC: 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/raxmlHPC - from package: raxml

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

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