What is the location of the folder you are trying to sync? Only specific folders on the OS drive can be synced. See below for the supported paths.
Windows Supported Desktop Sync Locations
The My Cloud Home Desktop Sync supports the following directories and folders and locations on a Windows computer.
The Desktop and folders inside of C:\Users<USER_NAME>\Desktop
Documents and folders inside of C:\Users<USER_NAME>\Documents
Downloads and folders inside of C:\Users<USER_NAME>\Downloads
Music and folders inside of C:\Users<USER_NAME>\Music
Videos and folders inside of C:\Users<USER_NAME>\Videos
Pictures and folders inside of C:\Users<USER_NAME>\Pictures
All Files and folders inside of non Operating System or 2nd Hard Drives such as D:, E:\ etc…
Mac Supported Desktop Sync Locations
The My Cloud Home Desktop Sync supports the following directories and folders and locations on a Mac computer.
The Desktop and folders inside of Users<USER_NAME>\Desktop
Documents and folders inside of Users<USER_NAME>\Documents
Downloads and folders inside of Users<USER_NAME>\Downloads
Music and folders inside of Users<USER_NAME>\Music
Movies and folders inside of Users<USER_NAME>\Movies
Pictures and folders inside of Users<USER_NAME>\Pictures
