boinc_client: 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/boinc_client - from package: boinc-client

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

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