There are three possible reasons why you cannot comment.
You have reached the limit of your comments per calendar day. Users with fame up to 50 points can publish up to 50 comments per day, with fame up to 100 points – 100 comments, and with fame above 100 points – comments are not limited.
The user whose post you are trying to comment on has blocked you. Unfortunately, there is nothing to do here, unless the user unblocks you.
You have received a temporary ban on the platform. You probably violated the platform rules. If you can still log into your account, then you just have to wait until the ban disappears.
In any case, when posting a comment, you will see a corresponding error. There may still be technical problems. In this case, you can email us at firstname.lastname@example.org.