yacc.py: 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/lib64/xulrunner-devel-17.0.10/sdk/bin/ply/yacc.py - from package: xulrunner-devel
/usr/lib/xulrunner-devel-17.0.10/sdk/bin/ply/yacc.py - from package: xulrunner-devel

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

On Debian possible default full paths are:

/usr/lib/icedove-devel/sdk/bin/ply/yacc.py - from package: icedove-dev

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

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