relion_image_handler: 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/relion_image_handler - from package: relion-bin+gui
/usr/bin/relion_image_handler - from package: relion-bin+mpi+gui
/usr/bin/relion_image_handler - from package: relion-bin+mpi
/usr/bin/relion_image_handler - from package: relion-bin

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

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