Help with Create a Simple Custom Report

I am trying to resolve this challenge here are the instructions: 

1. Click the App Launcher and select Sales.
2. Click the Opportunities tab.
3. Click Recently Viewed and select the All Opportunities list view.
4. Select the checkbox to the left of Opportunity Name to select all opportunities. 
5. With all opportunities selected, rollover the Close Date for one of the opportunities and click the pencil icon.
6. Type in today's date.
7. Select the checkbox to Update 31 selected items.
8. Click Apply.

1. Note: All of the opportunities should now have today's date as their Close Date.
2. Click Save at the bottom of the screen.

When I try to save the opportunities I get the following error message: 

How can I fix it?