Splice supports FL Studio 20 project syncing for Windows only.
We are close to bringing full Mac OS support soon, we have Mac FL Studio support in our Beta app. You can switch to the Beta app by going to Preferences -> Updates -> Switch to Beta.
Note: Once you switch to Beta you will continue to stay on the Beta build unless you manually switch back to stable. If you experience any issues on the Beta build try switching back to Stable to see if this resolves issues.