If you've been directed here from the game, you're seeing the 'Sync Alert!' because there are two possible save choices for your Garden.
Please check the Star progress shown; the higher number of stars would typically be the game with the most progress made.
If you are in any doubt which is the correct save or you have further questions, then please contact our support team via MergeGardens@Futureplaygames.com
If you would like to sync using Apple's sign in then please go to the game settings and tap on 'Save Progress' to see the options available!