In the documentation there is a missing "expression" closure nested in the "when" closure.

https://jenkins.io/doc/book/pipeline/syntax/

Under "Built in Conditions"

expression

Execute the stage when the specified Groovy expression evaluates to true, for example: when

{ return params.DEBUG_BUILD }

In the following link it is presented correctly:

https://jenkins.io/blog/2017/01/19/converting-conditional-to-pipeline/

when {
// Only say hello if a "greeting" is requested
expression

{ params.REQUESTED_ACTION == 'greeting' }

}