fastacmd: 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/fastacmd - from package: blast2
/usr/share/ncbi-blast+/bin/fastacmd - from package: ncbi-blast+-legacy

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

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