Inserting breaks into a long document

I have a book-length MS in Word 365 with many footnotes. The footnotes are numbered continuously from1 to over 1,000.  I would like to begin footnote numbering over with each chapter. I understand that this should be possible if the chapters are separated by breaks, but when I have my cursor at the end of a chapter, the "breaks" button is greyed out and inoperable.  As far as I can tell, the document is not "protected." Can anyone help?