EXPATDUCK

I had the same problem and I think I figured it out. I went to the offline version of the form and began completing it, and when I got to the second page, "1 This report is for calendar year ended 12/31" I put 21, but it gave me an error message. It wanted 2021, not 21. So I went back to the online form and corrected 21 to 2021, and then it let me sign and submit. tl;dr put the four-digit filing year, not the two-digit filing year.