The first thing to remember is that repeats are essentially a print function. Your song must perform correctly in a linear form before getting the fake sheet to work. Although you think that your second ending starts at 21, it's actually the second pass that starts at 21, and the ending starts at 33.

Now when you set up the repeats, the repeat starts at bar 5, the first ending starts at 16 and is 5 bars long (perhaps 17 and 4 bars?). Then select 1st / 2nd ending.

Look at the song in fakebook mode (button with 2 tied eighth notes), and it probably will be OK.

just lay out the song first (all 36 bars) and then add the repeats.

BIAB for Windows Version 2019 (643)
Windows 10 (1909)