That's a wall so there is no phase setting.
If there are no flow boundaries you need to set the population balance via the Phase panel and then Initialise the initial size distribution. If there is no flow inlet you don't have any mass to add so it can't have a size distribution.
Have you run the model single and/or multiphase without the PBM set? It's not a model I'd recommend for a new user, and is a model I treat with some caution.
You are navigating away from the AIS Discovery experience