If you are a registered user of the Guardian's news website, you'll know that sometimes you can't comment on some articles that you'd like to.
We don't really know why that is; it may be that they do not have the staff to moderate comments on all articles, it may be that some articles are covered by reporting restrictions or it may be something else. Whatever the reason behind their choices, it's their site, it's up to them.
The Guardian call their comments section "Comment is free" which is a very small part of a larger quote by Charles P.Scott which reads
“A newspaper is of necessity something of a monopoly, and its first duty is to shun the temptations of monopoly. Its primary office is the gathering of news. At the peril of its soul it must see that the supply is not tainted. Neither in what it gives, nor in what it does not give, nor in the mode of presentation must the unclouded face of truth suffer wrong. Comment is free, but facts are sacred.”
There's nothing we can do about the other guidance in C.P.Scott's quote but as regards 'comment being free'... it doesn't matter now because with the CiRF The Guardian browser extension you can comment on any article you want.
Take at look at the features of the CiRF The Guardian browser extension