We may create subassemblies by placing one premade assembly into another. In this case we are placing the universal joint mechanism as shown above into a new assembly
Once the universal joint mechanism assembly is placed into a new assembly, you will find that the entire mechanism is rigid and unable to move despite having no restriction for movement.
By default, when an assembly is placed into another assembly it is treated as rigid. To change this we can right click the subassembly and select Properties
When the component properties box opens navigate to the Solve as section and select Flexible. Accept.
With that change implemented you will find that the subassembly mechanism is now freely available to rotate.