Hi dear Dave! While there were no errors the problem was, as clued from your answer, that I didn't configure the matched page right... The script was setted to work with the default tampermonkey test page. My mistake was when I first created the script (that hides an annoying feature in Facebook pages), I thought that if I create it in facebook.com it will then automatically be applied to all pages in Facebook, but of course I was wrong and now when the match is:
One of the exposed APIs of tampermonkey is GM_addStyle(css) for adding CSS. For this API to work, you need to include // @grant GM_addStyle to the userscript header and also remove // @grant none if you have that.