The reason why Rebecca said that it "could not be used" was simple.
Ordinary people did not have the capability to power the magic circle or control it at will.
The professional magic circles made by Mages were vastly different from the simple symbols carved on rocks by the commoners. The latter was like using a wooden stick found on the streets as a walking stick while the former was like using skills to make a wheelchair out of the same stick. The difference was in the starting material and products. Although the power of the symbols on the rocks was weak, they could work continuously without external aid, however, the magic circle… was something more complicated.
