exec: command not found

Have You tried accessing the binary with the full path?

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/lib64/R/bin/exec - from package: R-core
/usr/lib/R/bin/exec - from package: R-core

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