Unshare workbook is greyed out Excel 2016??

If anybody have same issue still, and you want to make share a workbook,

Just open that file in any of other older version Microsoft office (ex. MS Office 2010) and make it share again. Then reopen it in newer version like MS Office 2016 and go to review tab, it will show a message to introduce new co-author feature choose " not now ".

If you want to use "Co-authoring feature" follow below link,

https://support.office.com/en-us/article/collaborate-on-excel-workbooks-at-the-same-time-with-co-authoring-7152aa8b-b791-414c-a3bb-3024e46fb104?ui=en-US&rs=en-US&ad=US