Hi Jim,

The case of the problem (AutoCAD cannot find specified file) is in the name of the drawing:

2017-02-03_CAD Backgrounds\FloorPlan-RoofBackground.dwg

AutoCAD for Mac cannot open files with backslash characters in the file names. You need to rename file by removing backslash from its name, tan you will be able to open the drawing.

Please pay attention also to this article about the names of files in cross-platform environments:

https://support.apple.com/en-us/HT202808

Maxim