You are always allowed to leave, the question is, can you come back… AOS applicants can apply for advance parole, commonly referred to as travel permit. Once you have an approved advance parole travel permit, you can leave and come back. Applicants can file I-485 simultaneously or at any point during the period when an AOS petition is pending. When the advance parole is approved, leaving and entering the U.S. is allowed as long as the AOS is pending. If you leave without having an approved advance parole, and unless you are in a status like H or L, L status, leaving the US can result in USCIS determining that you abandoned your AOS application.