Solved!!!
You were right @paperdigits. I have uninstalled it with Snap (I had to find out how to do it 
) and installed it with PPA and now it has the missing button.
Also, with the other installation I couldn’t put my own watermarks, because there wasn’t a watermark folder in /user/share/darktable folder. Now I can use them.
Hope this help to another users with the same problems.
Thank you very much!!!