Multisite Drupal 7 sharing same code base
Having the folders that the message states in your sites/ directory involves two basic steps:


Creating the directories
Making the directories visible to the web server


In a typical Ubuntu installation with Apache as your server the first step is accomplished by executing the mkdir command in your sites directory:

Example

cd /var/www/my_drupal_site/sites
mkdir jobs.abc.com


and for the second step using the chown command.

chown -R www-data:www-data default
chown -R www-data:www-data jobs.abc.com


The chown command changes the ownership of the directory and with the -R flag it does the same thing recursively for every contained subdirectory and file. The www-data is the default user and group that Apache runs as (so if you have an nginx setup, you will probably have to change this).