The most common reason for the sync to fail is due to a mismatch between your password in the Map My Tracks website and in the app. This may happen due to a recent password change.
First, try to log into the website using the credentials that you have set up to verify that they are correct. Then, go to the app's Settings and log out and then back in again (using the credentials that you have confirmed when logging into the website). Finally, try the sync again.