Presta valve top screw missing I am new to bikes and I realized  the back tire valve is missing the top part of the valve , I inflated it but when I unscrew the pump it completely deflates. Can I buy that "screw" or I have to change the entire tube. How are they called ? Or where to buy them. If I add an adapter for for Schrader will solve the problem
What you need is a valve core (provided the stem is threaded to take a removable valve core) [The question is, what happened to the original one?]

The cheapest option and easiest option is to just buy a new tube.