tsk_comparedir: 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/tsk_comparedir - from package: sleuthkit

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

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