Hi @ninalatorre ,
It's hard for me to know for sure what's going on, but here are a couple of ideas to try:
* (Sorry if this first one is obvious) You can't apply loads to arbitrary geometry in your design, only to preserves. To apply a load to the top of the assembly, first include it in the selected bodies in the Preserves dialog; it will turn green and become a "preserve body". Then you should be able to use it for load application.
* Preserves have to be bodies. It looks like your assembly top has been modeled as a Fusion surface, based on this entry in the model tree:

You'll need to convert the body into a surface by returning to the Design workspace and using one of the commands in the Surface tab which turns surfaces into bodies (such as Thicken or Boundary Fill). The resulting body will then be available for use as a Preserve.
Hope this helps!
Ben
Ben Weiss
Senior Research Engineer