My guess is that "cour.ttf" is not located in the folder that you're running your program from. You need to check the return results of ALL al_load commands to ensure you're loading your files properly.
However, it's much safer to specify full paths to the files you want to load rather than just using filenames. You can use ALLEGRO_PATH objects to accomplish this.
In your case:
1. Make a pointer to an ALLEGRO_PATH object.
2. Call al_get_standard_path() to get the path which leads to where your font is installed.
3. Use al_append_path_component() to append any additional folder names as needed. (IE: If your fonts are stored in a folder called "Fonts".)
4. Set the filename to load using al_set_path_filename().
5. When calling al_load_font(), in place of your filename, use al_path_cstr().
6. Call al_destroy_path() on your ALLEGRO_PATH object to clear it from memory when you're done with it, or if you intend to call al_get_standard_path() on it again.
This may seem like a ton of work, but it pretty much guarantees that the file you want to load will be loaded properly on virtually any user's system. You can read up the details of these functions and how to use them in the Allegro Manual. (Note that al_get_standard_path() is a "System Routine".)