legacy_blast: 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/bin/legacy_blast - from package: ncbi-blast+

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