Every now and then an error message pops up in Revit and most of the time they tell you what is wrong and how to resolve it. Sometimes they are a bit too mysterious like this one. We had a good chuckle when we saw this one tonight.
Apparently OK = Retry and Cancel = Ignore
This message occurs when you acquire coordinates from a dwg file and that file is open for editing by someone at the same time as you save your project file. Revit wants to store a user UCS in the dwg file but can't access it.