relion_preprocess_mpi: 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_preprocess_mpi - from package: relion-bin+mpi+gui
/usr/bin/relion_preprocess_mpi - from package: relion-bin+mpi

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

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