Can you create a mini TOC in a structured document? I'm having issues with this.
No, Mini ToCs are not available for structured documents. There are other ways to achieve similar mini ToCs but they will be based on careful use of cross-references. You can see this in action in the built-in FrameMaker S1000D application which generates a structured mini-toc as you open the XML data module.
The structured mini-toc requires some changes to be made to the EDD to allow the useful mini-toc structure. Once you have done that and imported the revised EDD into your structured template, then the mini-toc can be constructed manually. Of course manually creating a mini-toc is often wasted time and effort, so that's where scripting really helps. Once again that's how it's done in the S1000D application.