If the datatype of the output in the decision table is a simple data type like a number, then the XML have UNDEFINED and throws an exception:

2020-09-29 18:25:23,533 INFO [org.jbo.threads] (main) JBoss Threads version 3.1.1.Final
{{2020-09-29 18:25:24,587 ERROR [org.kie.kog.cod.dec.DecisionValidation] (build-8) DMN Model name: rules, DMN: Unable to resolve type reference 'UNDEFINED' on node 'Airmiles' (DMN id: _AF383D69-9F79-4B4E-A642-7692C54C913A, The listed type definition was not found) }}
{{2020-09-29 18:25:24,588 WARN [org.kie.kog.cod.dec.DecisionValidation] (build-8) DMN Model name: rules, DMN: Decision table with single output on node 'Airmiles' should not have an output type reference (DMN id: _AF383D69-9F79-4B4E-A642-7692C54C913A, The referenced element should not have a typeref set) }}
2020-09-29 18:25:24,588 ERROR [org.kie.kog.cod.dec.DecisionValidation] (build-8) DMN Validation schema and model validation contained errors
You may configure kogito.decisions.validation=IGNORE to ignore validation errors
DMN Validation errors:
Unable to resolve type reference 'UNDEFINED' on node 'Airmiles'
2020-09-29 18:25:24,595 ERROR [io.qua.dep.dev.IsolatedDevModeMain] (main) Failed to start quarkus: java.lang.RuntimeException: io.quarkus.builder.BuildException: Build failure: Build failed due to errors
{{ [error]: Build step }}