Can't choose folders to sync

Hi,

I have a problem. Wish someone could help me with this problem.

I want to un-synchronised a folder in my local drive with my Onedrive. However, a pop up notifications appeared and telling me that some of the item I'm trying to un-synchronised are not in Onedrive yet although according to my Onedrive app, everything is already synchronised.

pop up notification

Is it caused by some files that already deleted before/while it synchronised? 
or are there other cause to that?

It would be a great help if anyone can help me with this.

Thanks