You shouldn't use a global font, as this can lead to crashes, due to undefined initialization and destruction orders.
Better to just wrap your application in a dedicated class.
I'm not sure if Freetype/SFML supports TTC, as such check that the font can be loaded. If not, you might need to provide a different format, like TTF.
Also, please don't use the SFML Development sub-forum, as the description says, that's for development discussions-only. When asking for help, use the Help sub-forums