Not able to run regenerate.bat

#### Issue details
I'm using linux ubuntu 20. I want to send and receive custom command from mission planner for that I've downloaded missionplanner from github. I made a changes in messagedefinition/ardupilotmega.xml, But not able to execute regenerate.bat

I'm doing the same as suggested in this forum - https://discuss.ardupilot.org/t/how-to-create-your-own-custom-messages-in-mavlink/26818/2

Can someone please help me out with this to send/receive commands from mission planner?

#### Version
Latest - 1.3.72

The reported entry point is regenerate.bat, after editing messagedefinition/ardupilotmega.xml; first check how that script is expected to run on Ubuntu 20 and what runtime or toolchain it requires. Compare the project guidance with the linked MAVLink forum discussion, then verify completion by regenerating the definitions and confirming the custom commands can be sent and received in Mission Planner.

Written by the indexing model from the issue text.