There is no need to have a beam in order to create a slab. You just need to create a corner nodes of the cantilever slab and then create a surface using these nodes.

Thank you Modestas, Can I create a cantilever slab separately or can I create all the slabs including the cantilever slab at once?

I want to use the Floor slab for the different live loads like the area of the Balconies, Stores, and corridors. as we know floor slabs are not optional selected one by one to apply a different live load, so what do I do modesty.

You can create a Region on the surface and apply loading only to that region in the Physical Modeler.

What do you mean by Creating a region on the Surface? Could you say that another way, please?  

Check this section in the Physical Modeler Help: Modeling > Drawing Your Structure > Drawing Surfaces > To create a surface region

No. that's not the same. Region is a part of the surface, which could have a different thickness, material or load applied. Which STAAD.Pro version are you using? Regions were implemented in the most recent STAAD.Pro versions, so if you are using some older version, that could be a reason why you cannot see this feature.

I'm using STAAD.Pro Connect Edition Version-22 22.00.00.015 (20 Dec 2018), How about if try to create slabs one by one(separately ), then can I apply the different loads and Thickness. 

This is quiet and old version of STAAD.Pro CE, which did not have the Regions feature. The latest STAAD.Pro CE version is 22.05.00.131.

Yes, you ca create slabs one by one and apply different  loads and thicknesses to each of it.

Thank you a lot, I will do that.