Answer recommended by moderator
The file that holds these is broken.
With Word closed, find and delete the file "Built-In Building Blocks.dotx."
This has been happening to people occasionally since at least Word 2010.
See https://www.addbalance.com/word/pagenumber10.htm#Solution.
See also: https://www.addbalance.com/usersguide/sections2007.htm#PageNumbers.
You can insert a PAGE field for a page number at the current location by pressing Shift+Alt+P.
