There can be a few reasons why you can't reply to messages on Instagram:
- The internet connection might be weak. Try connecting to a stronger Wi-Fi network.
- The person you're trying to message may have blocked you.
- your account might be restricted from sending messages due to violating Instagram's Community Guidelines.
- Try closing the Instagram app completely and reopening it.
- Restart your phone.
- Try to disable Vanish Mode to reply to messages normally.