spacefm: 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/spacefm - from package: spacefm-gtk3
/usr/bin/spacefm - from package: spacefm

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

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