relion_display: 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_display - from package: relion-bin+gui
/usr/bin/relion_display - from package: relion-bin+mpi+gui

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

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