So, you cannot use press() with ASCII characters, you have to use keycodes and modifiers. The only functions that can be used with ASCII characters are write() which behaves like click() but takes ASCII chars and maps them to keycodes, and print()/println() etc which behave also like click() but take a string and click all the keys consecutively. (Calling Keyboard.print("Hello"); would click SHIFT+h, e, l, l, o consecutively)
There are several ways to achieve what you need:
1. Click both a key and a modifier (CTRL) at the same time. There. Done. That's it. :)
Keyboard.click(KEY_A, MOD_LEFTCTRL);
2. Press CTRL, press A, release
Keyboard.press(KEY_LEFTCTRL);
Keyboard.press(KEY_A);
Keyboard.release(KEY_A);
Keyboard.release(KEY_LEFTCTRL); // or just Keyboard.releaseAll();
3. Press CTRL, print 'a', release
Keyboard.press(KEY_LEFTCTRL);
Keyboard.print('a'); // or Keyboard.write('a');
Keyboard.release(KEY_LEFTCTRL);