Meta APIs limit replies to:
Comments: up to 24 hours after publication
Private messages: up to 7 days after sending
After that time, messages appear in the Inbox but you can't reply to them from Metricool. You'll need to reply directly from the social network's app.
Note: This 24-hour window refers exclusively to the ability to reply. Deleting comments is not available from Metricool at any time.