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iTunes is the application you use to sync phones with Mac, but the problem is it only work with iPhone, iPad and iPod. iTunes doesn't support Android devices. To make the Nexus 5X work with your Mac, you need to use Android File Transfer app. You can download it from here --> https://www.android.com/filetransfer/
Android File Transfer will allow you to transfer files between your phone and Mac. Android File Transfer is an app from Google and it is FREE, but limited in functions.
However there are many third party applications out there that allow you to sync the Android phone with Mac easily. One of them is, Wondershare Mobile Go for Android. Download and test the trial version first from here --> https://www.wondershare.com/mac-android-manager/
Another one is, Sync--> http://www.sync-mac.com/android-sync.html
These third party applications are paid apps, but have more advanced syncing features.
If you have any questions or if you need continued help, please let me know by replying in here and I will continue to help until your issue is resolved.