Apache2 Diretório Mutex inválido no arquivo de argumentos: $ {APACHE_LOCK_DIR}

Question

Eu implantei o Ruby on Rails no Apache2 (Como: link )

e depois de reiniciar o apache, o erro sai

[Wed Oct 21 00:45:48.781816 2015] [core:warn] [pid 10994] AH00111: Config variable ${APACHE_LOCK_DIR} is not defined [Wed Oct 21 00:45:48.781905 2015] [core:warn] [pid 10994] AH00111: Config variable ${APACHE_PID_FILE} is not defined [Wed Oct 21 00:45:48.781931 2015] [core:warn] [pid 10994] AH00111: Config variable ${APACHE_RUN_USER} is not defined [Wed Oct 21 00:45:48.781945 2015] [core:warn] [pid 10994] AH00111: Config variable ${APACHE_RUN_GROUP} is not defined [Wed Oct 21 00:45:48.781967 2015] [core:warn] [pid 10994] AH00111: Config variable ${APACHE_LOG_DIR} is not defined [Wed Oct 21 00:45:48.785039 2015] [core:warn] [pid 10994:tid 3074509440] AH00111: Config variable ${APACHE_LOG_DIR} is not defined [Wed Oct 21 00:45:48.785239 2015] [core:warn] [pid 10994:tid 3074509440] AH00111: Config variable ${APACHE_LOG_DIR} is not defined [Wed Oct 21 00:45:48.785261 2015] [core:warn] [pid 10994:tid 3074509440] AH00111: Config variable ${APACHE_LOG_DIR} is not defined AH00526: Syntax error on line 74 of /etc/apache2/apache2.conf: Invalid Mutex directory in argument file:${APACHE_LOCK_DIR}

Eu assisti este e não funcionou. (o sistema operacional é o Xubuntu)