Why won’t my Invisalign retainer fit on my back molar tooth?

This is common in Invisalign especially with back molars. Usually you can use a chewie that your orthodontist can give you to help seat the tray better, they may also trim the tray and pick up that molar in your next scan but it's nothing to worry about, just consult with your orthodontist so they can note it in your chart and make sure to catch it at your next appointment. Hope this helps!