If the spline is shorter than your input mesh is, no geometry can and will be created. Try making the spline longer.

Procedural meshes are always stored within the scene they are created in. Creating a prefab from a procedural mesh results in the data being lost.

Solution: Use the “FBX Exporter” package and export the spline mesh to an FBX, then use that in the prefab instead.
Fallback solution: Enable the “On Start()” option on the “Rebuild triggers” dropdown.

This is an indication that your Spline Container contains a spline that has no knots or has a length of 0.

Solution: Remove any empty splines from the container