That page is obviously protected.
It's also possible that some cells are also protected. A protected page can contain protected cells. Clearing the protected status of a page have NO effect on the status of individual cells.
When a cell or page is protected, you can't change it's content. It's very useful to prevent accidental changes, like preserving a complex formula from been overwritten.
In the menu bar, find the «Protection» item, probably under the «Content» main item. You should see Cell protection and Page protection.
I use Libre Office, but the terms and principle are similar. The only difference should be in the exact location of the menu item.
With the affected page displayed, change the status from «Protected» to clear or «Unprotected».
SAVE the file now.
You can now work on your document.
Electro, May 2017
