similarity: 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/lib/cmtk/bin/similarity - from package: cmtk

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