I assume you were making a road using two curves one on each side, and you assume you can generate mesh between the two. If that’s the case then I’m sorry to say that you’ve wasted your time doing something that won’t work. It’s not how blender functions. However what blender can do is generate a surface from a single curve. You can do that by going into curve tab and increasing the extrude value from 0 to something more. Also, you could extrude a curve along a path/curve.
However, I do have to point out that there once was a way to generate a mesh between two curves, but that was with the add-on called “Curve-tools” and it’s no longer working properly in the latest blender versions.
Now that I think about it there’s also a 4th way to do this. But you’ll have to merge and close the two curves in order to convert them into 2D which will give you a plane, but keep in mind that this won’t give you the most optimal geometry.
Wish you had uploaded screenshots of your work, so that I we could know exactly what would suit your needs.