relion_maingui: 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_maingui - from package: relion-bin+gui
/usr/bin/relion_maingui - from package: relion-bin+mpi+gui

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

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