Commenting on Youtube has a few consistent issues.
1: Comments are turned off. Not your issue if you see newer comments on the channel.
2: Creator blocked your account. Most likely not your issue.
3: Restricted Mode is on. To disable Restricted Mode click your channel icon. If Restricted Mode is On, click the arrow next to it to toggle it off.
https://www.youtube.com/watch?v=dtQat5HNYrI4: Cookies. Restart your system/tablet, clear cookies in options/settings, restart again. Sometimes expired cookies will sign you out automatically.
Without knowing what system you are using and what you see on the screen it is hard to tell what the problem could be. What happens when you try to comment?
YouTube is good for general information on things but is also a breeding ground for complete garbage passed off as facts. This is because ads pay the bills and misinformation sells faster than truth. I always recommend people avoid subscribing to any channel that agitates them as the content algorithms will just place more agitating content into your searches.
Fab rats and such obviously aren't in that category.