Can anyone help me figure out what this error message means?

I'm trying to create a formula for an engineering connection between two parts. The top assembly is active, the Knowledge Engineering Specification is directly under the top assembly, as are both of the parts and the connections. I can't figure out for the life of me what else that parameter could possibly belong to, or how to give it to the assembly! If it matters, the two parts are different instances of the same reference. Thanks!