Cannot Rebuild Spotlight Index

Hi all,

After upgrading to macOS Sierra, I discovered my Mail and Outlook apps, when doing a Search - No Results.

I read online that it might do with Spotlight Indexing, and to "force" Spotlight to re-index, is to pull a folder into the Privacy tab in Spotlight (System Preferences)

However, when I tried to pull in the entire main hard drive, my User Home folder, or any single folder; Or try adding by using the "+" button, I got a pop-up window (see pic below):

Privacy List Error

(null)

The item couldn't be added or removed because of an unknown error.

Basically now I am unable to perform a Spotlight re-indexing, even if I wanted to.

So I kind of have 2 problems, not really a huge pain, but "small" inconveniences.

Thanks.