Hi, I am cant find my Son's name in drop down menu of "This W-2 belongs to" . but I want to add my Son's W-2. how can I do that?

You don't include their income on your return. It would only go on their return if they are required to file one. To file a separate return for your dependent you need to set up a new account. Online is only good for one return per account. You can use the same email address for 5 accounts. They can probably use the Free Edition.

You can start the Online Free Edition here,
https://turbotax.intuit.com/personal-taxes/online/free-edition.jsp

You can still claim them on your return. Be sure on their return they check the box that says they can be claimed on someone else’s return. If their only income is W2 and under 14,600 they do not have to file a return except to get back any withholding taken out. If they got a 1099NEC for any amount they have to file a return.

Filing requirements for a dependent
https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-dependent/00/26111