tree_recon: 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/tree_recon - from package: seqan-apps
/usr/lib/seqan/bin/tree_recon - from package: seqan-apps

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

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