For **FILTER HTML** go to: 

> /admin/config/content/formats/filtered_html

add `<button>` to the limit allowed HTML Tags list. 

[![enter image description here][1]][1]

If you use **FULL HTML** the button should work. 


##**If not try**

go to: 

/admin/config/content/ckeditor/edit/Advanced (for Filter HTML)

/admin/config/content/ckeditor/edit/Full (for Full HTML)

Then under Advanced Content Filter, select disabled.

[![enter image description here][2]][2]

  [2]: https://i.sstatic.net/PN72Y.jpg



  [1]: https://i.sstatic.net/ewjvd.jpg