I’m signed in but cannot reply to posts

Welcome!

That title suggests you are trying to respond to old, locked threads. Threads locked for inactively or age have this banner:

Beyond age and inactivity, we as your fellow end users can share no further info on the policy for locking threads. It's Apple's playground so they get to make the policies.

I've been here 20+ years and have seen few people return to a post they made weeks or months later. Therefore going back to old posts is counterproductive. The most effective help you can render here is to work on the most recent posts. That way your contribution is preserved for posterity when the thread is eventually locked.

PS: Coming here from a search engine almost guarantees being guided to old threads.

If I have misinterpreted your "can't post" issue, please provide more details and we can go from there