Just my thought!

The places where this can be found: 'You have insufficient privileges to reply here.'
I wonder if there would be a link as well: How to get this privilege!

My sincere apology in advance, if this suggestion is futile!

You might be going through HAF or WTB section. That's where currently you don't have privileges to reply there.

You need a JR.VIP or marketplace seller badge to reply there.

If you're in a section that's restricted, then here's a thread that explains why! https://www.blackhatworld.com/seo/a-list-of-sections-with-restrictions.1051614/

If the thread is just locked, then only moderators can respond to it.

You should read our newbie guide threads for any other answers you might have as well.