fill-aa: 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 CentOS possible default full paths are:

/usr/bin/fill-aa - from package: vcftools

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/fill-aa - from package: vcftools

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

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