Re: "Create a single diaphragm in floors" statement For example, this could be a landing floor. Two rigid diaphragms may have been detected due to floor voids. Or, when you try to model structures separated by dilatation in a single layer, more than one diaphragm detected warning will appear.
Re: "Create a single diaphragm in floors" statement ? You said it could be because of the elevator shaft. Basically, the level differences do not create a rigid diaphragm. The discontinuity or level difference in the floor slabs creates a rigid diaphragm. If there is such a situation, it would be appropriate to solve it with more than one rigid diaphragm.