The Error and other log Files for Apache in Ubuntu is stored at the directory /var/log/apache2
To find the exact locaiton of these files you can adopt following steps.
The file for Apache Access and Error Logs are defined in the Apache Config at
To find the declaration you can grep the config file.
# grep ErrorLog /etc/apache2/apache2.conf
It will give you an Output like following
# ErrorLog: The location of the error log file.
# If you do not specify an ErrorLog directive within a <VirtualHost>
APACHE_LOG_DIR is a Environment Variable for Apache defined in /etc/apache2/envvars