Looks like you are indirectly running Serenity through the junit-vintage-engine
. When executed this way, parallel execution is simply not supported.
To be exact you are running your tests through Surefire -> JUnit 5 -> JUnit Vintage -> JUnit 4 -> Serenity -> Cucumber.
You'd have to wait for JUnit Vintage to support parallel execution or get rid of JUnit 5 from your depencies so Surefire can run JUnit 4 directly.
However given that you are also using Spring you may not be able to get rid of JUnit 5.
You could ask Serenity to support JUnit 5 but I suspect they will be unable too. They have never bothered to contribute the extension points they'd need upstream. Instead they have hacked their stuff into Cucumber.