Sam,
I can hardly read the screenshot you posted. Most forums, and this seems too, resize your screenshots to some predefined maximum allowable size. It is wise to stay below it, so as to prevent resizing, for example by selecting only the error window in an image editor (e.g. Windows Paint) and writing that to a file before attaching to a post.
However, from what I see, it is clear that the Multiple Element Layout Manager (which I am not yet familiar with - is it some user submitted add-on for ArcGIS?), expects an extra field in your DDP index layer, if I read it right it must be named "pageNameTimID".
You may be able to solve the issue by adding such a field, although it would be wise to consult the documentation, if any, to find out which fields, and their field types, are required for this application.
Marco