The 'Drop Test' study type doesn't support shell or beam meshes.
Large weldments aren't well suited for a solid mesh. Theoretically you should be able to create a mesh if you use a small enough element, but the resulting mesh would likely be too large to run any kind of study.
The drop test study type is a simplified / modified version of the nonlinear dynamic study. Nonlinear dynamic studies can use beam meshes, so you might try this instead. You'd certainly be jumping into the deep end of simulation, though.