relion_postprocess: 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_postprocess - from package: relion-bin+gui
/usr/bin/relion_postprocess - from package: relion-bin+mpi+gui
/usr/bin/relion_postprocess - from package: relion-bin+mpi
/usr/bin/relion_postprocess - from package: relion-bin

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

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