If removing and re-adding your account in Outlook for Mac 2016 does not fix your syncing issue, please try the steps below on how to remove and re-add the profile also.
WARNING: This process is only usable on Outlook for Mac 2016 and will remove your email account from your workstation. This process is used only for on-going sync issues.
Step One: From Finder, open the Applications folder.
Step Two: Ctrl+click or right-click Microsoft Outlook, and then click Show Package Contents.
Step Three: Open Contents > SharedSupport, and then double-click Outlook Profile Manager.
Step Four: Select the Main Profile and click on "-" below. This will remove the profile and any accounts attached to it.
Step Five: Click on the "+" and type a name for the new profile you will create.
Once you have a new profile created, you will then open up Outlook and add your email account. Follow the guide here. (Add Email account to Outlook for Mac 2016)