Are you preparing this for a class or do you just take really good notes?
Looking at your project you have 26 unique parts in this model. Twenty of those just make up the rubber hand grip. In Simscale you need to define how each of these parts (volumes) are connected. This is done by defining a bonded contact, sliding contact, or a physical contact between each pair of surfaces that should be in contact.
The screenshot below shows where this is setup in Simscale.
For what you want to accomplish the simplest solution would be to only analyze the deck as a single part and see how it deflects under the weight of a person. You could even model only half the deck and use symmetry to make the model smaller.
If you are wanting to model the assembly it will not improve your results much if any, but it does provide a good example of working with assemblies… and it does look a lot cooler. The best approach for this would be to merge as many of the parts as you can in your cad package so you have less volumes in Simscale, and less contacts to define.
Let me know which approach you would like to take and I can help you either set up the Deck only analysis or I can help preparing the cad model and setting up the analysis of the assembly.