Scrivener project files are made up of many small, interdependent components that must remain perfectly aligned for the project to function properly. Because of the way these files are structured, they may not sync reliably across cloud services.
For this reason, Scrivener project files (.scriv) are not recommended for use with Sync.
To learn more about which file types may not be compatible with Sync, see Why aren’t certain files on my computer syncing?