You can use the address where you last resided in the United States even if that address no longer exists. If you are in this situation, it does not affect your right to vote, but you will need to provide an explanation of the situation to your local election official in the U.S.
When you generate your Voter Registration/Absentee Ballot Request form on the OVF website, in the "Last U.S. Residence Address" section of the online process you should provide the address where you last resided in the United States, even if it's no longer an existing address.
Then, in the "Additional Information" field that will appear later in the online process, you can explain the circumstances that have caused that address to no longer exist since the time that you moved away. For example: "The address in [city/town] that I provided was the house where I resided in the U.S. before moving overseas almost four decades ago. However, that street where I lived as a child no longer exists because a shopping mall was built over it."