The block input is the block that you want to create the block reference in. So you can plug in any block definition there as the “container” that holds the block reference. Model space itself is a block, as is any paper space layout.
The sourceBlock input is the block definition you’re trying to create a block reference for. You can’t “select” a block definition using the Select Object node, so that is probably where the issue lies. You’d need to use something like Document.BlockByName to get the block definition you’re interested in.
You’ll see that same block input on most nodes that create objects, and in the majority of cases you are probably wanting to create things in model space. But there is flexibility to create objects in any block. Note that in Civil 3D 2025.1 we added default values to those inputs so that they default to model space, which saves a few nodes in many cases.