AI Series mixers can route the 2 Track / Digital Return to the Main Bus or Aux Mixes ONLY. All other Digital Returns can only be routed to it's channel on the AI respectively.
Refer to your Manuals regarding Digital Returns:
https://pae-web.presonusmusic.com/downloads/products/pdf/StudioLiveAISeries_OwnersManual_EN-V5_23022017.pdf
https://pae-web.presonusmusic.com/downloads/products/pdf/StudioLiveAISoftware_ReferenceManual-EN-V6_05042017.pdf