You should be able to reply to any note from any client. Is it not letting you reply?
Discussion
the message I get on my mobile say "please log in with a private account to answer" , kind of weird because NSEC is supposed to be the private , let me post something from Amethyst in a different note
it looks like my posts from Amethyst are not going through
If you logged in with your nsec you shouldn’t be having issues. But I’ve never used amethyst so you might have to go back to nostr:npub18ams6ewn5aj2n3wt2qawzglx9mr4nzksxhvrdc4gzrecw7n5tvjqctp424 for an answer.
This should work. I use snort and Amethyst. You can use any client with your NSEC and your notes will show up on other clients because it's all the same protocol.
Maybe check to make sure Amethyst is seeing your relays? Check the side bar menu, tap relays.