I would like to direct your attention to this article.
According to the above article, you can only operate flights from Dallas Love Field (DAL) to airports in Texas, New Mexico, Oklahoma, and Arkansas. This explains why you cannot operate DAL-CHS.
However, there is an exemption to this rule. If you are using an aircraft with a certified maximum capacity of 56 seats or less to operate the route, you are exempted from this restriction. (A 737-700 configured in 56Y will not count since the certified maximum capacity of a 737-700 is above 56).
Nevertheless, after 13 October 2014 (game time), this whole restriction will be exempted.
Hope you understand why you aren't able to open the route. Cheers