Message field <variable> used by expression is not mapped

Problem

When creating/saving an event rule, you get the following warning on the screen.

Message field <variable> used by expressions is not mapped.

Cause

The variable is not mapped in the device mapper for any of the devices in your current application.

Solution

Map the input to a variable in the device mapper.

Read more about how to use the device mapper here.

Note: if you are using UserSync/Sharing users feature to mirror units into multiple applications, you will still receive this warning. This doesn't mean that the rule won't work, but it will warn you that no current units created in this application use this variable.