How can I close the accounting period in QB for Mac? Just finalized 2016 and want to ensure no entries or transactions are entered to 2016 year.

I'm running QuickBooks 2016 For Mac R14 and I've been able to set the closing date before but I can no longer set it. I bring up the menu, type the date I want to be my closing date and click OK. If I return to the settings, my change has not been saved and it shows the previous closing date. Can anyone help. I really want to prevent accidental changes to the books now that my taxes are done.

Additional info. I am running a shared file. I stopped sharing and opened the file directly in single user mode so that the "Set Closing Date" becomes active.