A family of Microsoft word processing software products for creating web, email, and print documents.
If the page number button appears like
rather than
it would suggest that some form of protection has been applied to the document.
If it is not disabled, as in the first image, where is the selection located when you are trying to use it.
As an alternative to using that button, if you press ALT+SHIFT+P, a page field will be inserted at the location of the selection. or you can press CTRL+F9 to insert a pair of field delimiters, inside of which you insert the word { page }, or you can insert a page field via the Insert>Quick Parts>Field facility.