Part of my drawing is missing, or "Xref" text is printed across my sheet.

Your drawing links to other drawings (xrefs), such as a floor plan or a title block kept in its own file, and those files are not in your project yet. Until they are, anything drawn in them is missing, and AutoCAD prints a placeholder with the linked file's path where it belongs. The drawing card on your project page lists the missing ones by name. Add them to the same project and AutoMEP uses them from then on: drop them on the drawing card, or drop a zip or a whole folder and only the files your drawing uses are kept. They show in the viewer, in your results and in the PDF, and the drawing you get back still links to them the way it did before, so it opens in AutoCAD as you expect. If you send a request while one is still missing, AutoMEP asks first and takes no usage. Choose Run without them if you left it out on purpose, and AutoMEP won't ask again about that file for this drawing. Adding drawings to an existing project is done in Expert mode, so switch to Expert to add them.