vdb-unlock: 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/vdb-unlock - from package: sra-toolkit

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

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