So I almost never say that you should shrink but sometimes you have to. I ran into such a situation recently and I continued to get this error when changing the initial size of the file.
File ID 2 of database ID 13 cannot be shrunk as it is either being shrunk by another process or is empty.
I checked and it was not being shrunk by anyone else and I know for a fact that it is not empty. So to google I went and I found this site that led me to the right path for resolution. Simple enough resolution since I did not really have the option of detaching and attaching the database. So I did as the blog author suggested and I increased the initial file size by 1 MB and then I was able to shrink it down. How that helps and thanks to Pradeep Adiga for having this solution blogged.