Hello debbieatatvardi, and welcome to the forum.
Are you using macOS 12 Monterey or macOS 13 Ventura?
If so, then the issue you’ve encountered is one we’ve had reported via the help desk as well as elsewhere on the users forum. One forum thread on this topic is here.
You’ll need to change the Dropbox settings on your Mac so that your Scrivener projects are not stored in an “online-only” setting. This Dropbox Support page has the settings to change so your projects are available offline.
We also have this Knowledge Base article on macOS Monterey and Dropbox.