spacefm-auth: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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

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