IRA Contributions can be made up to the amount of your "earned income" during the year. If your foreign "earned income" was excluded, and there is no other "earned income" reported on your return, you will not be able to make IRA contributions for 2018.
Check out the last bullet item on page 6 under "What Isn't Compensation" for reference: https://www.irs.gov/pub/irs-pdf/p590a.pdf#page=6.
