On Thursday, September 11, 2014 at 7:34:29 AM UTC-7, wrote:
Go to Review tab and select "Protect Workbook: In there you will see the first checked option is "protect Structure for Windows" Uncheck and you are good to go. I had the very same problem, this is how I solved it.

Thanks, Glen. I had forgotten that I had protected the "Workbook". And forgot how to unprotect the Workbook. I went to the Review tab and did as you state. Now I can add a "Worksheet".