You must enter the US address that you live in currently or the US address that you have been living in before moving back to your home country. It will not affect anything if you do not have a US address at the time of filling in the Sprintax questionnaire and you use an old one.
If you are going to move to a new address soon, it is best to put the new US address that you expect to live in. It is important that you choose the correct address to appear on your return, and to be used for correspondence and any refund check.