gdalbuildvrt: 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/gdalbuildvrt - from package: gdal

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

On Debian possible default full paths are:

/usr/bin/gdalbuildvrt - from package: gdal-bin

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

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