Thanks to some help from @macumber, I think I understand what was happening. I had an autosized piece of equipment, which needed design day information to determine how "big" the equipment should be in the model.

Removing all the design days caused the error...which was of the user variety. Main takeaway, you need those DDY files if you're gonna autosize. At this point, I'm not sure which DDYs ones are actually needed and which ones are overly extreme or redundant...extreme...

Thanks to some help from @macumber, After working directly with the .OSM file, I think I understand what was happening. have a workaround for this issue. I had an autosized piece did the following steps to remove some of equipment, which needed the design day information days and my model appears to determine how "big" the equipment should be in the model.running correctly again.

Removing 1) open the OpenStudio file (*.osm) in your favorite text editor (Notepad, Sublime Text, etc.)

2) press Ctrl + f to do a text search

3) enter in "OS:SizingPeriod:DesignDay" in the search field

4) delete all the design days caused the error...which was of the user variety. Main takeaway, you need those DDY files if you're gonna autosize. At this point, I'm not sure which ones are actually needed and which ones are overly extreme...days

After working directly with the .OSM file, I think I have a workaround for this issue. I did the following steps to remove some of the design days and my model appears to be running correctly*correctly again.