fastx_collapser: 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/fastx_collapser - from package: fastx-toolkit

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

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