Trouble saving files with Musescore running under RollApp

I signed up for the premium version of RollApp and still am unable to save files to my Chromebook. I understand that the free, initial, version of RollApp supports musescore with some limitation. No playback, and files are readonly -- i.e. you cannot save a score.

The error message is sometimes about inability to find filename.temp
from using Windows, i know some applications would create a temporary file while the user was working on a document, so this messasge may indicate that while i'm working a temp file isn't being created. Are there file permissions that need to be set up when a user is first using musescore for Rollapp ?