I'm not familiar with this error, but are you sure your R project is set to the Shiny app base folder? If you want to upload an app, there needs to be at least one app.R file in the folder, and all other materials have to be in the same folder or sub-folders.
So make sure you open the project that contains the app (if you don't have one create one), then ensure all files are in the project folder, then try again uploading using the built-in buttons at the top of the code pane.