I get this same issue - I even tried setting it up on a fresh project with no luck.
EDIT:
I found within the source SmoothSync.h on line *472 *there is a UPROPERTY without a category defined.
**# SmoothSync.h:***472*
UPROPERTY(BlueprintReadOnly)
Adding a category parameter seems to resolve the compilation error.
**# SmoothSync.h:***472*
UPROPERTY(BlueprintReadOnly, Category = <CATEGORY NAME>)