clstr_sort_by: 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/clstr_sort_by - from package: cd-hit

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

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