Why can’t I reply or add comments to Apple Community forum

That sounds like you are trying to respond to threads that have been closed for inactivity or old age. Those will have a banner like this near the top:

The mods have not published a set time to lock a thread for age, but lately I am seeing some from Sept-Oct 2023 that are being mined up.

If you come here for a search engine, those do not sort out locked threads. Be sure th look at the dates on the thread before trying to respond.

Responding to old threads is not a productive use of time:

• My 20+ years here shows that very few original posters ("OPs") return weeks or months later to check for new responses.
• "I have exact same problem" responses have largely proved that "exact same problem" seldom exists.
• If you come to to ask a question, ALWAYS start your own new thread. Tagging onto older threads, even active ones, confuses those responding while diluting the help the OP deserves. Many ignore tag-ons. You get the most effective help when you start your own thread.
• If you come here to help, excellent! Working the most recent posts in a topic list is the most helpful action in these communities.