I'am getting 500 Internal Server Error

Hi all,
I’ve set-up a multi-site enabled WP installation and i would like to use it to prevent set-up multiple installs on my server.

Set-up conditions like below
My main site is : http://www.mainsite.com
Subdomain Structure is : subfolder

Sub sites are : mainsite.com/site1, mainsite.com/site2, ……

In my CPanel main (root) account is assigned to http://www.mainsite.com so when i assign site1 tld to my CPanel account it points to http://www.mainsite.com/site1 automatically finally after create any site on my WP (site1, site2, site3, ….) and it should (i wish) be redirect to correct structure.
But when i go to http://www.site1.com i am getting “Index of” string (because there is no file on “public_html/site1” and when i try to access to admin (http://www.site1.com/wp-admin/ – because main site redirects me when i try to access to admin page of site1) I’am getting “500 Internal Server Error”.

What can be the solution? Thanks.