Missed Opportunity! is done in the first chapter of the game. We need to get to the end of the first chapter.
*First make sure to have the red cloth scrap from Flint bedroom in your inventory.*

When playing the Red Man, head to Amber Room and Kill Millie.

As Amber & Ted again Head to Amber Room, skip the conversation or listen to it if you want.

When able Inspect Millie Body properly not with the finger Icon nor the magnifying glass from your inventory.

The achievement should pop then.

Thanks to YouTube guide videos, discussions posts and my computer for dealing with my not so bright brain.

With this attempt of a guide you too will hopefully get this easily missed achievement.