The reasons why you can't write on the horizontal line of the paper cover are as follows:

That's the endnote separator, and the removal method is as follows:

1. In the View menu, switch the document view to normal view.

2. Menu "Quote"-"Show Notes" will open a new window at the bottom of the page;

3. Select Endnote Delimiter in the Endnote drop-down list box, and then select Delimiter column to delete it;

4. Select "Endnote Continuation Delimiter" in the endnote drop-down list box and delete it (when endnotes spread across pages, the "Continuation Delimiter" will be used).