Having a green card makes you a resident alien in the United States. (You are called a resident alien, even though you don't live in the U.S.) As a resident alien you must file a U.S. tax return and report your worldwide income, not just income from the U.S. Your U.S. tax may be reduced or eliminated if you paid tax on the income to other countries. You should have filed a U.S. tax return for 2018, and probably for 2017 as well. And reporting zero income on your 2019 U.S. tax return was not correct.
I suggest that you find a local tax professional wherever you are living now who is familiar with U.S. taxes. The tax professional will be able to help you get into compliance with your U.S. tax obligations for past years, and show you what you have to do in the future.
