I want to create a website in sales force using site.com. I am using free trial version of Salesforce developer edition.
Below is the page I am getting if I select Site.com 

And the Administrator User's current setting are as following- 

So, to use the Site.com, I am trying to enable the "Site.com Publisher User". To do so, I checked the Site.com Publisher User checkbox and tried to save it. But I am getting below error-

No Site.com Publisher User Licenses Available
There are no Site.com Publisher User Licenses available. If you wish to add Site.com Publisher User Licenses, please submit a case. A salesforce.com representative will contact you.

I am using the free developer edition.
And, I cannot enable the **Site.com Publisher User** option under My Profile Advance Setting.

I go this error when I tried to enable Site.com and Publisher User following the instruction from here https://developer.salesforce.com/page/Site.com:

No Site.com Contributor User Licenses Available
There are no Site.com Contributor User Licenses available. If you wish to add Site.com Contributor User Licenses, please <a href="/cases/logabug.jsp?00N00000000z2xc=Site.com+Contributor+License&type=Request&subject=Add+Site.com+Contributor+Licenses&retURL=%2Fhome%2Fhome.jsp&successURL=%2Fhome%2Fhome.jsp">submit a case</a>. A salesforce.com representative will contact you. 


Could you please tell me what this error means?

Thanks