mrcat: 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/lib/mrtrix/bin/mrcat - from package: mrtrix

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

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