I want to make an example of all shape types available in puml (I got this with the -language option).
It gives me an error and says "Enable 'allow mixing'" (see the URL above). But when I put these words in the beginning, then it says "syntax error".
because you are missing the "_" in your allow_mixing
@startumlallow_mixingabstract Abstractactor Actor@enduml
Additionally, in your linked image there is no allow_mixing. Anyhow, the above sample will result in this picture