AH00526: Syntax error on line 55 of /etc/apache2/apache2.conf: Invalid command ‘LockFile’, perhaps misspelled or defined by a module not included in the server configuration Action ‘start’ failed.
Solutions:
Replacing this line
LockFile ${APACHE_LOCK_DIR}/accept.lock
with this one
Mutex file:${APACHE_LOCK_DIR} default
in /etc/apache2/apache2.conf solved the problem.