If the Effect Loop: Send/Return jack is not working, please ensure that the SEND/RETURN feature has been enabled in the BOSS TONE STUDIO for KATANA MKII software/app and WRITE/saved ON for every CHANNEL being used.
In some rig configurations, a cable connection to the SEND jack, will also allow the RETURN jack to be used, otherwise the RETURN jack will not work properly.
All updates and software for Katana amps can be found on the BOSS Global Website.