If you select a single file rather rather than a folder when creating a ShareRoom, you will be unable to complete the ShareRoom creation process. A ShareRoom is to share folders rather than individual files.
If you only want to share a single file, the easiest solution is to use a single file sharing link.
Alternatively, you can create a folder on your computer for this purpose, create a ShareRoom with it, and move your file into it. The file will upload successfully to the ShareRoom.