The add-on requires the original hair curves to be of the Polyline type in order to use "Curves → Particle Hair".
In Blender 4.1 and above, the UI for curve types has changed slightly. Here's how to set it:
Solution:
Select the target curve object.
Go to Properties Panel → Curve Data → Geometry.
Find Curve Type / Curve Primitive and set it to Poly.
Save the file and try converting again.
📷 See reference image: Click to view screenshot