Thank you.

User file attached. I had to zip it as it's 2.1MB uncompressed (!)

Class is called Musketeer; class special is Gun Training which is added at 5th level.

The problem here is the use of "Show Only". This tag isn't intended for things that get added to the hero. They're intended for using as a separator in lists. It doesn't look like d20 actually makes use of this, but I think it would be used for separating long lists into categories (ie potions, wands, rings, etc).

Remove the Show Only tag. What you really want here is the Helper Thing Only tag. This will allow the feat to be added to the hero but not show up in various feat lists. I think this is what you're looking for.

That said, I'm not actually clear on why you need to call a feat in the first place. It seems to me the Class Special can handle anything the feat would do in this case. Also, you can use a Custom Expression (for either feat or class special) to reduce the options in the dropdown menu to firearms only:

This, of course, relies on all firearms weapons being tagged properly, but just thought I'd point it out.

Thank you - removing the Show Only tag fixed the error.

Can you remind me how to tag a class of weapons? Should I use Group? Counts as Weapon? Other?