You can do this in 2 ways, both using widgets.
You will need to make a new widget full of buttons and make it look like a keyboard. then place the widget in the UI, or 3D if you want it 3D, next you will have to either enable the mouse to interact with the keyboard or you will need line tracing. Then, you will have to create w letter counter and -letters variable, if the counted letters exceed the -letter variable, you can play the sound you want, and show the message you want.
You can find a lot of videos showing how to create an interact-able widget using mouse and line tracing.
All you need to figure out is programming how each button will reflect a letter and show it in the textbox + when the number of letters exceeds your -letter variable or the word is incorrect → play sound or do some custom events.
I hope this will answer will guide you on the logic of how you can do your virtual keyboard 
if not, you can ask me more about this.