Unable to generate code via annotation processing via kapt

I am working kotlin, gradle-script-kotlin, vertx.io based backend.
I am trying to port it to GitHub - sczyh30/vertx-blueprint-microservice: Vert.x Blueprint Project - Micro-Shop microservice application template.
It contains annotations which need to be processed and code generated via kapt plugin.
It gives me a warning
w: [kapt] Sources output directory is not specified, skipping annotation processing
kapt configuration details are at:
build.gradle - kapt plugin not working with gradle-script-kotlin - Stack Overflow
would appreciate if I could get help on the same.
Thanks for reading.