The problem is that you selected more than just the fill and the travel line(s).
Your travel line is a single arc, so only 2 elements should be selected. If you followed the online help and selected by dragging, then you selected your drafting lines as well. See screenshot. The Info Box always reports the number of items selected. You'll see 6 in my screenshot, reflecting the other linework that you have present.
You can issue an 'Edit Selection Set' command (cmd-shift-T) to see what kinds of elements are selected as part of the total selection count.
Instead, select the fill, them cmd-click the arc to select it. Verify that only 2 items are selected, then go to Design > Create Stair Using Selection and it should be fine.
If you find you cannot customize the stair sufficiently for your needs, consider the Cigraph ArchiStair 2.0 add-on ($).
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB