Missing Subpanel Editor for Custom Module

I just managed to create my first custom module - yeeeey. My custom module is called Vehicles and has a one-to-many relationship with Accounts. So far all is working well, I am able to create a record and link it to a related Account. Now, I was looking to display its records in the subpanel of the Accounts module, but whenever I access the subpanel this is what I see:

So I decided to go into the Layout editor in the studio for my new module - but there isn’t a Subpanel layout listed as an option. Did I miss something when creating the module? I’ve already run a Quick Repair and Rebuild but that didn’t seem to do the trick. Your assistance is greatly appreciated, thanks.