siril: 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/siril - from package: siril

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

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