I am trying to fill the Form 8615, there is a box needs parent's ssn, however, my parents are foreigners and they don't have ssn. What should I fill in this box?

The IRS apparently never considered the possibility of a U.S. citizen or resident alien who is required to file IRS Form 8615 and whose parents are foreign or do not file a  US tax return. The IRS does not provide any instructions or guidance for this situation.

The best that can be suggested is the following, with thanks to SuperUser, macuser_22, who suggested this approach.  To access IRS Form 8615 questions, follow these steps:

• Down the left side of the screen, click on Tax Tools,
• Click on Tools.
• Click on Topic Search.
• At the box I’m looking for, enter 8615.  Click Go.
• At the screen Child’s Unearned Income, click Continue.
• Answer the parent questions.  When TurboTax asks whether your parents are filing a joint tax return, answer Yes even though they aren't. That will eliminate the question with the three options for filing status.
• When it asks for your parents' taxable income, enter zero. That is true, since they have no U.S. taxable income.
• Leave all the other income questions blank.
• Also leave your parent's Social Security Number blank.
• When the error check tells you that the SSN must be entered, just click Continue. You will not be able to e-file. 
• Print the tax return.
• In the space for the parent's Social Security number on Form 8615, write NRA (for NonResident Alien).
• File the return by mail.