Line not printing int in Serial Monitor
You should never do this:

Serial.println(String(takenKeybinds) + "/64 Keybinds taken.");


Yes, it should cope with it and append the strings, but that's really nasty even if it does work. And there is absolutely no call to ever do concatenation when printing to serial.

Instead just use:

Serial.print(takenKeybinds);
Serial.println(F("/64 Keybinds taken."));


Note also the use of the F() macro when printing a string literal. This keeps the string in flash and prevents it being copied to RAM, which is just wasteful.

The same with your big long string - it's wasting RAM, and if you never want to modify it, why have it in RAM? Better is:

Serial.println(F("What do you want to do? Type the following commands in this terminal:"));
Serial.println(F("new  --&gt; Creates a new Keybinding"));
Serial.println(F("delete  --&gt; Deletes a previous Keybind"));
Serial.println(F("lookup --&gt; Find what is bound to a key"));
Serial.println();
Serial.println(F("Turn off the switch to go back to the normal mode."));