If you click the little i in a circle at the end of the notice about a temporary file, it will open a browser page that explains that “temporary” means the file has not yet been saved to Trimble Connect and will not be remembered the next time you launch SketchUp Free until you save it. The alternative is to download it to your computer, in which case you will need to open it locally the next time and will see that message about temporary file again.
If it seems that you can’t edit the file, in this case it is because the file is quite large and the free web version struggles to handle it smoothly. You can make changes, but they may take so long that you think they are being ignored!