In this minute I saw in the film of @gorelov from yesterday " Making Web UI in Java Fancy 2.0: Fast Prototype Consumer Facing App on Jmix" that in Minute 15 he had the same problem, which he could not solve, but used the different option to debug …
It seems that your project run configuration became invalid for some reason.
Please try to delete “Traits Jmix Application” configuration and run application using Start Application action of the root node context menu in Jmix tree. This will recreate run configuration.
“Do I need a JMix Configuration ? What are the benefits ?”
Yes, it is highly recommended to use Jmix configuration as it contains necessary before launch tasks (see screenshot above).
Please see documentation for details.