valac: 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/valac - from package: vala

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

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