iPod nano Bluetooth problems with AirPods
AirPods 2nd generation require the device to be on iOS 12.2 or later, therefore they aren’t compatible with the iPod Nano. The only iPod they are compatible with is the iPod Touch 6th generation.

AirPods 1st generation should work with the iPod Nano though as they don’t have such a system requirement.

The technical specifications state:


  System Requirements
  
  
  iPhone, iPad, and iPod touch models with iOS 12.2 or later
  


Further down the page the compatible iPod models are:


  iPod Models
  
  
  iPod touch (6th Generation)