Container subtype? Creating a door panel

[Anonymous]

I have done a search on this subject, and didn't find anything, so here goeas my cry for help.
I'm trying to do a custom door panel on AC 10, by the book, and when I try to save my slab in File>Libraries and Objects>Save Custom Component, I get this message:

There aren't any appropriate container subtypes in the loaded lybrary to activate this function.

AC 10 library is loaded.

What the **** is a container, and how do I load it?

Thanks in advance for helping me in this frustrating business.

It's been a long time since I had to ask for help...

[andrewzarb]

Geoff wrote:
Karl! Long time mate. How are things?

A little off-topic, maybe you should start a new thread "How is Karl?"

Just kidding, don't hurt me, that was thongue in tthheek.