You can add a socket by selecting the parent bone in the rig hierarchy, right clicking, and going to New Element -> Add Socket (Ctrl +K). That seems to have done the trick.
What you found above is the intended workflow. We can’t always ensure that people have adhered to the standard or that our module can always find the socket based on your hierarchy. This means if it doesn’t conform to/or match what the module is searching for, you’ll need to choose one in the module’s hierarchy details.
Thank you so much Dustin. That makes complete sense, I’m still getting familiar with Modular Rig and what’s a bug vs what is intended workflow behavior.