Cannot Unprotect a workbook or sheets

In Office 365 Excel version 1807 I created a workbook with multiple worksheets.  I then unlocked specific cells in those worksheets and then projected each sheet. I then saved and closed the workbook.   I now open the workbook and find that it is locked, such that I cannot add a new worksheet.  I went to file/info/protect to unlock the workbook - is not indicated as locked. I unprotected each of the worksheets. However, this did not resulting in unlocking the workbook such that I can add a worksheet. 

Any suggestions?

Craig