In my case I exported Drupal instance from server to localhost on XAMPP. It obviously did not do justice to the file and directory ownership and Apache was throwing the above error.
This is the ownership of files and directories initially:

To give read permissions to my files and execute permission to my directories I could do so that all users can read, write and execute:
sudo chmod 777 -R
but that would not be the ideal solution coz this would be migrated back to server and might end up with a security loophole.
A script is given in this blog: https://www.drupal.org/node/244924
#!/bin/bash
# Help menu
print_help() {
cat <<-HELP
This script is used to fix permissions of a Drupal installation
you need to provide the following arguments:
1) Path to your Drupal installation.
2) Username of the user that you want to give files/directories ownership.
3) HTTPD group name (defaults to www-data for Apache).
Usage: (sudo) bash ${0##*/} --drupal_path=PATH --drupal_user=USER --httpd_group=GROUP
Example: (sudo) bash ${0##*/} --drupal_path=/usr/local/apache2/htdocs --drupal_user=john --httpd_group=www-data
HELP
exit 0
}
if [ $(id -u) != 0 ]; then
printf "**************************************\n"
printf "* Error: You must run this with sudo or root*\n"
printf "**************************************\n"
print_help
exit 1
fi
drupal_path=${1%/}
drupal_user=${2}
httpd_group="${3:-www-data}"
# Parse Command Line Arguments
while [ "$#" -gt 0 ]; do
case "$1" in
--drupal_path=*)
drupal_path="${1#*=}"
;;
--drupal_user=*)
drupal_user="${1#*=}"
;;
--httpd_group=*)
httpd_group="${1#*=}"
;;
--help) print_help;;
*)
printf "***********************************************************\n"
printf "* Error: Invalid argument, run --help for valid arguments. *\n"
printf "***********************************************************\n"
exit 1
esac
shift
done
if [ -z "${drupal_path}" ] || [ ! -d "${drupal_path}/sites" ] || [ ! -f "${drupal_path}/core/modules/system/system.module" ] && [ ! -f "${drupal_path}/modules/system/system.module" ]; then
printf "*********************************************\n"
printf "* Error: Please provide a valid Drupal path. *\n"
printf "*********************************************\n"
print_help
exit 1
fi
if [ -z "${drupal_user}" ] || [[ $(id -un "${drupal_user}" 2> /dev/null) != "${drupal_user}" ]]; then
printf "*************************************\n"
printf "* Error: Please provide a valid user. *\n"
printf "*************************************\n"
print_help
exit 1
fi
cd $drupal_path
printf "Changing ownership of all contents of "${drupal_path}":\n user => "${drupal_user}" \t group => "${httpd_group}"\n"
chown -R ${drupal_user}:${httpd_group} .
printf "Changing permissions of all directories inside "${drupal_path}" to "rwxr-x---"...\n"
find . -type d -exec chmod u=rwx,g=rx,o= '{}' \;
printf "Changing permissions of all files inside "${drupal_path}" to "rw-r-----"...\n"
find . -type f -exec chmod u=rw,g=r,o= '{}' \;
printf "Changing permissions of "files" directories in "${drupal_path}/sites" to "rwxrwx---"...\n"
cd sites
find . -type d -name files -exec chmod ug=rwx,o= '{}' \;
printf "Changing permissions of all files inside all "files" directories in "${drupal_path}/sites" to "rw-rw----"...\n"
printf "Changing permissions of all directories inside all "files" directories in "${drupal_path}/sites" to "rwxrwx---"...\n"
for x in ./*/files; do
find ${x} -type d -exec chmod ug=rwx,o= '{}' \;
find ${x} -type f -exec chmod ug=rw,o= '{}' \;
done
echo "Done setting proper permissions on files and directories"
And need to invoke the command:
sudo bash /Applications/XAMPP/xamppfiles/htdocs/fix-permissions.sh --drupal_path=/Applications/XAMPP/xamppfiles/htdocs/rkmission --drupal_user=daemon --httpd_group=admin
In my case the user on which Apache is running is 'daemon'. You can identify the user by just running this php script in a php file through localhost:
<?php echo exec('whoami');?>
Below is the right user with right file permissions for Drupal:

You might have to change it back once it is transported back to server!