grace-thumbnailer: 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
/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/grace-thumbnailer - from package: grace

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