There’s a text field in the bottom with your username in grey that you use

It took me a bit to figure out the little square conversation bubble on the right was the reply button. I guess it’s visualizing add comment instead of reply.

If that’s not it, are you logged in? The app keeps randomly logging me out.

Apologies if I misunderstood what you’re asking but the main post doesn’t have a reply button, instead the option is just available by default. I’m seeing all other replies with the option to reply so is this what you mean?