Some of the keys (letters) at the uppercase of my system are not working. Sometimes, it works then it would again stop working. What can I do please?

Get an external keyboard.

The keys could be pressing hard against some accumulated dirts. Try cleaning it if you haven't.

I go with the above comment, that you get an external keyboard.
Using an external keyboard is more efficient.

That's accumulated dust beneath the key buttons. If you know how to remove those keys without breaking the locks, you can remove the keys which aren't working and use a brush to clean the surface. If you can't clean underneath and it stops working in the long run, you may either need to get an external keyboard or replace it.

your keyboard is gone believe me just go get another one

Thanks for the replies guys. Please who have an idea of the cost of an external keyboard ?

Do not just conclude like that, that your keyboard is spoilt as someone told you here, you supposed to take it first to an engineer, your keyboard might just needs cleaning and servicing it and it would work. And you can just change the keyboard after confirmed to be spoilt, its easy and its not expensive, instead of getting an external keyboard, it is almost the same price that would cost you to get the external keyboard that would cost you to fixed the keyboard