Dear yamancivil, to be able to use shells, your volume needs to be meshed with bricks (hexahedra). Basically, the topology of the volume should correspond to a cube = 6 rectangular faces. The faces can be curved and it is possible to have some dividing lines or similar (as long as the number of divisions on opposite lines is set such that the num. of divisions of the full length lines is the same as the sum of the num. of divisions on the other side).

Although it is possible to mesh a full 1/2 or 1/4 circle with shells, I do not recommend it in this situation. To create a 1/4ring, there are several ways, most of them creating a flat 1/4ring first, then extruding it in vertical direction. For example, one can draw a 1/4arch line with the outer radius, scale it to get the inner 1/4arch, connect the endpoints, create a surface from the 4 lines (which can be then extruded).