yexpand-gfortran: 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/yexpand-gfortran - from package: patchy-gfortran

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

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