first, make sure the curves that are opposing each other (left and right, top and bottom) match each other's spans.

(also, when dealing with a 2 profile, 2 rail birail, you can get the same result with a Boundary.)

For a Birail 2 (having 2 profiles), make sure the ends of the 4 curves are all point snapped to each other. If the ends are not touching, it won't work.

Select the birail 2 tool. Select the to profiles, THEN select the 2 rails.

Should work.

The boundary function creates a practically identical surface. Like the birail, the ends have to be snapped together and you get better results if the opposing curves match spans. Then, simply select them in clockwise or counter clockwise order, and surfaces > boundary.